diff --git a/examples/ppo_cartpole.py b/examples/ppo_cartpole.py
new file mode 100755
index 0000000..6fc986f
--- /dev/null
+++ b/examples/ppo_cartpole.py
@@ -0,0 +1,98 @@
+#!/usr/bin/env python
+from __future__ import absolute_import
+
+import tensorflow as tf
+
+# our lib imports here! It's ok to append path in examples
+import sys
+sys.path.append('..')
+from tianshou.core import losses
+from tianshou.data.batch import Batch
+import tianshou.data.advantage_estimation as advantage_estimation
+import tianshou.core.policy.stochastic as policy  # TODO: fix imports as zhusuan so that only need to import to policy
+
+from rllab.envs.box2d.cartpole_env import CartpoleEnv
+from rllab.envs.normalized_env import normalize
+
+
+def policy_net(observation, action_dim, scope=None):
+    """
+    Constructs the policy network. NOT NEEDED IN THE LIBRARY! this is pure tf
+
+    :param observation: Placeholder for the observation. A tensor of shape (bs, x, y, channels)
+    :param action_dim: int. The number of actions.
+    :param scope: str. Specifying the scope of the variables.
+    """
+    # with tf.variable_scope(scope):
+    net = tf.layers.dense(observation, 32, activation=tf.nn.tanh)
+    net = tf.layers.dense(net, 32, activation=tf.nn.tanh)
+
+    act_mean = tf.layers.dense(net, action_dim, activation=None)
+
+    return act_mean
+
+
+if __name__ == '__main__': # a clean version with only policy net, no value net
+    env = normalize(CartpoleEnv())
+    observation_dim = env.observation_space.shape
+    action_dim = env.action_space.flat_dim
+
+    clip_param = 0.2
+    num_batches = 10
+    batch_size = 512
+
+    # 1. build network with pure tf
+    observation = tf.placeholder(tf.float32, shape=(None,) + observation_dim) # network input
+
+    with tf.variable_scope('pi'):
+        action_mean = policy_net(observation, action_dim, 'pi')
+        action_logstd = tf.get_variable('action_logstd', shape=(action_dim,))
+        train_var_list = tf.get_collection(tf.GraphKeys.TRAINABLE_VARIABLES) # TODO: better management of TRAINABLE_VARIABLES
+    with tf.variable_scope('pi_old'):
+        action_mean_old = policy_net(observation, action_dim, 'pi_old')
+        action_logstd_old = tf.get_variable('action_logstd_old', shape=(action_dim,))
+        pi_old_var_list = tf.get_collection(tf.GraphKeys.TRAINABLE_VARIABLES, 'pi_old')
+
+    # 2. build losses, optimizers
+    pi = policy.Normal(action_mean, action_logstd, observation_placeholder=observation) # YongRen: policy.Gaussian (could reference the policy in TRPO paper, my code is adapted from zhusuan.distributions) policy.DQN etc.
+    # for continuous action space, you may need to change an environment to run
+    pi_old = policy.Normal(action_mean_old, action_logstd_old, observation_placeholder=observation)
+
+    action = tf.placeholder(dtype=tf.float32, shape=(None, action_dim)) # batch of integer actions
+    advantage = tf.placeholder(dtype=tf.float32, shape=(None,)) # advantage values used in the Gradients
+
+    ppo_loss_clip = losses.ppo_clip(action, advantage, clip_param, pi, pi_old) # TongzhengRen: losses.vpg ... management of placeholders and feed_dict
+
+    total_loss = ppo_loss_clip
+    optimizer = tf.train.AdamOptimizer(1e-4)
+    train_op = optimizer.minimize(total_loss, var_list=train_var_list)
+
+    # 3. define data collection
+    training_data = Batch(env, pi, advantage_estimation.full_return) # YouQiaoben: finish and polish Batch, advantage_estimation.gae_lambda as in PPO paper
+                                                             # ShihongSong: Replay(), see dqn_example.py
+    # maybe a dict to manage the elements to be collected
+
+    # 4. start training
+    config = tf.ConfigProto()
+    config.gpu_options.allow_growth = True
+    with tf.Session(config=config) as sess:
+        sess.run(tf.global_variables_initializer())
+        # sync pi and pi_old
+        sess.run([tf.assign(theta_old, theta) for (theta_old, theta) in zip(pi_old_var_list, train_var_list)])
+
+        for i in range(100): # until some stopping criterion met...
+            # collect data
+            training_data.collect(num_episodes=120) # YouQiaoben, ShihongSong
+
+            # print current return
+            print('Epoch {}:'.format(i))
+            training_data.statistics()
+
+            # update network
+            for _ in range(num_batches):
+                data = training_data.next_batch(batch_size) # YouQiaoben, ShihongSong
+                # TODO: auto managing of the placeholders? or add this to params of data.Batch
+                sess.run(train_op, feed_dict={observation: data['observations'], action: data['actions'], advantage: data['returns']})
+
+            # assigning pi to pi_old
+            sess.run([tf.assign(theta_old, theta) for (theta_old, theta) in zip(pi_old_var_list, train_var_list)])
\ No newline at end of file
diff --git a/tianshou/core/policy/base.py b/tianshou/core/policy/base.py
index 1c1e1c5..8d4b2a1 100644
--- a/tianshou/core/policy/base.py
+++ b/tianshou/core/policy/base.py
@@ -55,7 +55,7 @@ class QValuePolicy(object):
 
 class StochasticPolicy(object):
     """
-    The :class:`Distribution` class is the base class for various probabilistic
+    The :class:`StochasticPolicy` class is the base class for various probabilistic
     distributions which support batch inputs, generating batches of samples and
     evaluate probabilities at batches of given values.
 
diff --git a/tianshou/core/policy/stochastic.py b/tianshou/core/policy/stochastic.py
index d7a75d7..e2c2dea 100644
--- a/tianshou/core/policy/stochastic.py
+++ b/tianshou/core/policy/stochastic.py
@@ -62,9 +62,11 @@ class OnehotCategorical(StochasticPolicy):
         return self._n_categories
 
     def _act(self, observation):
-        sess = tf.get_default_session() # TODO: this may be ugly. also maybe huge problem when parallel
-        sampled_action = sess.run(tf.multinomial(self.logits, num_samples=1), feed_dict={self._observation_placeholder: observation[None]})
+        # TODO: this may be ugly. also maybe huge problem when parallel
+        sess = tf.get_default_session()
         # observation[None] adds one dimension at the beginning
+        sampled_action = sess.run(tf.multinomial(self.logits, num_samples=1),
+                                           feed_dict={self._observation_placeholder: observation[None]})
 
         sampled_action = sampled_action[0, 0]
 
@@ -73,28 +75,75 @@ class OnehotCategorical(StochasticPolicy):
     def _log_prob(self, sampled_action):
         return -tf.nn.sparse_softmax_cross_entropy_with_logits(labels=sampled_action, logits=self.logits)
 
-        # given = tf.cast(given, self.param_dtype)
-        # given, logits = maybe_explicit_broadcast(
-        #     given, self.logits, 'given', 'logits')
-        # if (given.get_shape().ndims == 2) or (logits.get_shape().ndims == 2):
-        #     given_flat = given
-        #     logits_flat = logits
-        # else:
-        #     given_flat = tf.reshape(given, [-1, self.n_categories])
-        #     logits_flat = tf.reshape(logits, [-1, self.n_categories])
-        # log_p_flat = -tf.nn.softmax_cross_entropy_with_logits(
-        #     labels=given_flat, logits=logits_flat)
-        # if (given.get_shape().ndims == 2) or (logits.get_shape().ndims == 2):
-        #     log_p = log_p_flat
-        # else:
-        #     log_p = tf.reshape(log_p_flat, tf.shape(logits)[:-1])
-        #     if given.get_shape() and logits.get_shape():
-        #         log_p.set_shape(tf.broadcast_static_shape(
-        #             given.get_shape(), logits.get_shape())[:-1])
-        # return log_p
 
     def _prob(self, sampled_action):
         return tf.exp(self._log_prob(sampled_action))
 
 
-OnehotDiscrete = OnehotCategorical
\ No newline at end of file
+OnehotDiscrete = OnehotCategorical
+
+
+class Normal(StochasticPolicy):
+    """
+        The :class:`Normal' class is the Normal policy
+
+        :param mean:
+        :param std:
+        :param group_ndims
+        :param observation_placeholder
+    """
+    def __init__(self,
+                 mean = 0.,
+                 logstd = 1.,
+                 group_ndims = 1,
+                 observation_placeholder = None,
+                 **kwargs):
+
+        self._mean = tf.convert_to_tensor(mean, dtype = tf.float32)
+        self._logstd = tf.convert_to_tensor(logstd, dtype = tf.float32)
+        self._std = tf.exp(self._logstd)
+
+        super(Normal, self).__init__(
+            act_dtype = tf.float32,
+            param_dtype = tf.float32,
+            is_continuous = True,
+            observation_placeholder = observation_placeholder,
+            group_ndims = group_ndims,
+            **kwargs)
+
+    @property
+    def mean(self):
+        return self._mean
+
+    @property
+    def std(self):
+        return self._std
+
+    @property
+    def logstd(self):
+        return self._logstd
+
+    def _act(self, observation):
+        # TODO: getting session like this maybe ugly. also maybe huge problem when parallel
+        sess = tf.get_default_session()
+        mean, std = self._mean, self._std
+        shape = tf.broadcast_dynamic_shape(tf.shape(self._mean),\
+                                           tf.shape(self._std))
+
+
+        # observation[None] adds one dimension at the beginning
+        sampled_action = sess.run(tf.random_normal(tf.concat([[1], shape], 0),
+                                  dtype = tf.float32) * std + mean,
+                                  feed_dict={self._observation_placeholder: observation[None]})
+        sampled_action = sampled_action[0, 0]
+        return sampled_action
+
+
+    def _log_prob(self, sampled_action):
+        mean, logstd = self._mean, self._logstd
+        c = -0.5 * np.log(2 * np.pi)
+        precision = tf.exp(-2 * logstd)
+        return c - logstd - 0.5 * precision * tf.square(sampled_action - mean)
+
+    def _prob(self, sampled_action):
+        return tf.exp(self._log_prob(sampled_action))
\ No newline at end of file
diff --git a/tianshou/data/batch.py b/tianshou/data/batch.py
index 7b28966..4d7b1f2 100644
--- a/tianshou/data/batch.py
+++ b/tianshou/data/batch.py
@@ -8,7 +8,7 @@ class Batch(object):
     class for batch datasets. Collect multiple observations (actions, rewards, etc.) on-policy.
     """
 
-    def __init__(self, env, pi, advantage_estimation_function): # how to name the function?
+    def __init__(self, env, pi, advantage_estimation_function):  # how to name the function?
         self._env = env
         self._pi = pi
         self._advantage_estimation_function = advantage_estimation_function
@@ -63,7 +63,7 @@ class Batch(object):
                     ob = env.reset()
                 t += 1
 
-        if num_episodes > 0: # YouQiaoben: fix memory growth, both del and gc.collect() fail
+        if num_episodes > 0:  # YouQiaoben: fix memory growth, both del and gc.collect() fail
             # initialize rawdata lists
             if not self._is_first_collect:
                 del self.observations
@@ -91,10 +91,10 @@ class Batch(object):
                     rewards.append(reward)
 
                     t_count += 1
-                    if t_count >= 200: # force episode stop, just to test if memory still grows
-                        break
+                    if t_count >= 100:  # force episode stop, just to test if memory still grows
+                        done = True
 
-                    if done: # end of episode, discard s_T
+                    if done:  # end of episode, discard s_T
                         break
                     else:
                         observations.append(ob)
@@ -122,7 +122,46 @@ class Batch(object):
     def apply_advantage_estimation_function(self):
         self.data = self._advantage_estimation_function(self.raw_data)
 
-    def next_batch(self, batch_size): # YouQiaoben: referencing other iterate over batches
+    def next_batch(self, batch_size, standardize_advantage=True):  # YouQiaoben: referencing other iterate over batches
         rand_idx = np.random.choice(self.data['observations'].shape[0], batch_size)
-        return {key: value[rand_idx] for key, value in self.data.items()}
+        current_batch = {key: value[rand_idx] for key, value in self.data.items()}
 
+        if standardize_advantage:
+            advantage_mean = np.mean(current_batch['returns'])
+            advantage_std = np.std(current_batch['returns'])
+            current_batch['returns'] = (current_batch['returns'] - advantage_mean) / advantage_std
+
+        return current_batch
+
+    def statistics(self):
+        """
+        compute the statistics of the current sampled paths
+        :return:
+        """
+        rewards = self.raw_data['rewards']
+        episode_start_flags = self.raw_data['episode_start_flags']
+        num_timesteps = rewards.shape[0]
+
+        returns = []
+        max_return = 0
+        episode_start_idx = 0
+        for i in range(1, num_timesteps):
+            if episode_start_flags[i] or (
+                    i == num_timesteps - 1):  # found the start of next episode or the end of all episodes
+                if i < rewards.shape[0] - 1:
+                    t = i - 1
+                else:
+                    t = i
+                Gt = 0
+                while t >= episode_start_idx:
+                    Gt += rewards[t]
+                    t -= 1
+
+                returns.append(Gt)
+                if Gt > max_return:
+                    max_return = Gt
+                episode_start_idx = i
+
+        print('AverageReturn: {}'.format(np.mean(returns)))
+        print('StdReturn:   : {}'.format(np.std(returns)))
+        print('MaxReturn    : {}'.format(max_return))
\ No newline at end of file