diff --git a/README.md b/README.md
index bc2414a..d07beae 100644
--- a/README.md
+++ b/README.md
@@ -106,6 +106,7 @@ Note: install openai/gym first to run the Atari environment; note that interface
 
 Without preprocessing and other tricks, this example will not train to any meaningful results. Codes should past two tests: individual module test and run through this example code.
 
-## Dependency
-Tensorflow (Version >= 1.4)
-Gym
+## Dependencies
+TensorFlow (Version >= 1.4)
+
+gym
diff --git a/examples/dqn_replay.py b/examples/dqn_replay.py
new file mode 100644
index 0000000..ca2351a
--- /dev/null
+++ b/examples/dqn_replay.py
@@ -0,0 +1,87 @@
+#!/usr/bin/env python
+from __future__ import absolute_import
+
+import tensorflow as tf
+import gym
+import numpy as np
+import time
+
+# 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.dqn as policy  # TODO: fix imports as zhusuan so that only need to import to policy
+import tianshou.core.value_function.action_value as value_function
+
+import tianshou.data.replay as replay
+import tianshou.data.data_collector as data_collector
+
+
+# TODO: why this solves cartpole even without training?
+
+
+if __name__ == '__main__':
+    env = gym.make('CartPole-v0')
+    observation_dim = env.observation_space.shape
+    action_dim = env.action_space.n
+
+    clip_param = 0.2
+    num_batches = 10
+    batch_size = 512
+
+    seed = 0
+    np.random.seed(seed)
+    tf.set_random_seed(seed)
+
+    ### 1. build network with pure tf
+    observation_ph = tf.placeholder(tf.float32, shape=(None,) + observation_dim)
+
+    def my_network():
+        net = tf.layers.dense(observation_ph, 32, activation=tf.nn.tanh)
+        net = tf.layers.dense(net, 32, activation=tf.nn.tanh)
+
+        action_values = tf.layers.dense(net, action_dim, activation=None)
+
+        return None, action_values  # no policy head
+
+    ### 2. build policy, loss, optimizer
+    dqn = value_function.DQN(my_network, observation_placeholder=observation_ph, weight_update=100)
+    pi = policy.DQN(dqn)
+
+    dqn_loss = losses.qlearning(dqn)
+
+    total_loss = dqn_loss
+    optimizer = tf.train.AdamOptimizer(1e-4)
+    train_op = optimizer.minimize(total_loss, var_list=dqn.trainable_variables)
+
+    ### 3. define data collection
+    replay_buffer = replay()
+
+    data_collector = data_collector(env, pi, [advantage_estimation.nstep_q_return(1, dqn)], [dqn], replay_buffer)
+
+    ### 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())
+
+        # assign actor to pi_old
+        pi.sync_weights()  # TODO: automate this for policies with target network
+
+        start_time = time.time()
+        for i in range(100):
+            # collect data
+            data_collector.collect(num_episodes=50)
+
+            # print current return
+            print('Epoch {}:'.format(i))
+            data_collector.statistics()
+
+            # update network
+            for _ in range(num_batches):
+                feed_dict = data_collector.next_batch(batch_size)
+                sess.run(train_op, feed_dict=feed_dict)
+
+            print('Elapsed time: {:.1f} min'.format((time.time() - start_time) / 60))
diff --git a/tianshou/data/advantage_estimation.py b/tianshou/data/advantage_estimation.py
index b9bf0e3..43351e1 100644
--- a/tianshou/data/advantage_estimation.py
+++ b/tianshou/data/advantage_estimation.py
@@ -158,4 +158,3 @@ class QLearningTarget:
         data['rewards'] = np.array(rewards)
 
         return data
-
diff --git a/tianshou/data/replay_buffer/naive.py b/tianshou/data/replay_buffer/naive.py
index 5eb4dd7..a253352 100644
--- a/tianshou/data/replay_buffer/naive.py
+++ b/tianshou/data/replay_buffer/naive.py
@@ -3,7 +3,7 @@ import tensorflow as tf
 from collections import deque
 from math import fabs
 
-from tianshou.data.replay_buffer.buffer import ReplayBuffer
+from .buffer import ReplayBuffer
 
 
 class NaiveExperience(ReplayBuffer):
diff --git a/tianshou/data/replay_buffer/rank_based.py b/tianshou/data/replay_buffer/rank_based.py
index 0a6641f..efa9041 100644
--- a/tianshou/data/replay_buffer/rank_based.py
+++ b/tianshou/data/replay_buffer/rank_based.py
@@ -1,9 +1,3 @@
-#!/usr/bin/python
-# -*- encoding=utf-8 -*-
-# author: Ian
-# e-mail: stmayue@gmail.com
-# description: 
-
 import sys
 import math
 import random
diff --git a/tianshou/data/replay_buffer/utils.py b/tianshou/data/replay_buffer/utils.py
index 4480375..aee0fef 100644
--- a/tianshou/data/replay_buffer/utils.py
+++ b/tianshou/data/replay_buffer/utils.py
@@ -1,8 +1,8 @@
 import sys
 
-from tianshou.data.replay_buffer.naive import NaiveExperience
-from tianshou.data.replay_buffer.proportional import PropotionalExperience
-from tianshou.data.replay_buffer.rank_based import RankBasedExperience
+from .naive import NaiveExperience
+from .proportional import PropotionalExperience
+from .rank_based import RankBasedExperience
 
 
 def get_replay_buffer(name, env, policy, qnet, target_qnet, conf):