diff --git a/.github/workflows/pytest.yml b/.github/workflows/pytest.yml
index 10a1551..33d166e 100644
--- a/.github/workflows/pytest.yml
+++ b/.github/workflows/pytest.yml
@@ -11,7 +11,6 @@ jobs:
     strategy:
       matrix:
         python-version: [3.6, 3.7, 3.8]
-
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}
diff --git a/test/base/env.py b/test/base/env.py
index d2fd671..5a2294b 100644
--- a/test/base/env.py
+++ b/test/base/env.py
@@ -8,9 +8,9 @@ class MyTestEnv(gym.Env):
         self.sleep = sleep
         self.reset()
 
-    def reset(self):
+    def reset(self, state=0):
         self.done = False
-        self.index = 0
+        self.index = state
         return self.index
 
     def step(self, action):
diff --git a/test/base/test_batch.py b/test/base/test_batch.py
index d5b22ca..53a2cdc 100644
--- a/test/base/test_batch.py
+++ b/test/base/test_batch.py
@@ -17,6 +17,7 @@ def test_batch():
     batch.obs = np.arange(5)
     for i, b in enumerate(batch.split(1, permute=False)):
         assert b.obs == batch[i].obs
+    print(batch)
 
 
 if __name__ == '__main__':
diff --git a/test/base/test_buffer.py b/test/base/test_buffer.py
index 38d3bc5..ba8a4d5 100644
--- a/test/base/test_buffer.py
+++ b/test/base/test_buffer.py
@@ -1,3 +1,4 @@
+import numpy as np
 from tianshou.data import ReplayBuffer
 
 if __name__ == '__main__':
@@ -28,5 +29,24 @@ def test_replaybuffer(size=10, bufsize=20):
     assert buf2[-1].obs == buf[4].obs
 
 
+def test_stack(size=5, bufsize=9, stack_num=4):
+    env = MyTestEnv(size)
+    buf = ReplayBuffer(bufsize, stack_num)
+    obs = env.reset(1)
+    for i in range(15):
+        obs_next, rew, done, info = env.step(1)
+        buf.add(obs, 1, rew, done, None, info)
+        obs = obs_next
+        if done:
+            obs = env.reset(1)
+    indice = np.arange(len(buf))
+    assert abs(buf.get_stack(indice, 'obs') - np.array([
+        [1, 1, 1, 2], [1, 1, 2, 3], [1, 2, 3, 4],
+        [1, 1, 1, 1], [1, 1, 1, 2], [1, 1, 2, 3],
+        [3, 3, 3, 3], [3, 3, 3, 4], [1, 1, 1, 1]])).sum() < 1e-6
+    print(buf)
+
+
 if __name__ == '__main__':
     test_replaybuffer()
+    test_stack()
diff --git a/test/discrete/test_drqn.py b/test/discrete/test_drqn.py
index 76a6106..f851d4e 100644
--- a/test/discrete/test_drqn.py
+++ b/test/discrete/test_drqn.py
@@ -26,7 +26,7 @@ def get_args():
     parser.add_argument('--stack-num', type=int, default=4)
     parser.add_argument('--lr', type=float, default=1e-3)
     parser.add_argument('--gamma', type=float, default=0.9)
-    parser.add_argument('--n-step', type=int, default=3)
+    parser.add_argument('--n-step', type=int, default=4)
     parser.add_argument('--target-update-freq', type=int, default=320)
     parser.add_argument('--epoch', type=int, default=100)
     parser.add_argument('--step-per-epoch', type=int, default=1000)
diff --git a/test/discrete/test_pg.py b/test/discrete/test_pg.py
index 5bcdeab..081f48a 100644
--- a/test/discrete/test_pg.py
+++ b/test/discrete/test_pg.py
@@ -31,27 +31,29 @@ def compute_return_base(batch, aa=None, bb=None, gamma=0.1):
 
 def test_fn(size=2560):
     policy = PGPolicy(None, None, None, discount_factor=0.1)
+    buf = ReplayBuffer(100)
+    buf.add(1, 1, 1, 1, 1)
     fn = policy.process_fn
     # fn = compute_return_base
     batch = Batch(
         done=np.array([1, 0, 0, 1, 0, 1, 0, 1.]),
         rew=np.array([0, 1, 2, 3, 4, 5, 6, 7.]),
     )
-    batch = fn(batch, None, None)
+    batch = fn(batch, buf, 0)
     ans = np.array([0, 1.23, 2.3, 3, 4.5, 5, 6.7, 7])
     assert abs(batch.returns - ans).sum() <= 1e-5
     batch = Batch(
         done=np.array([0, 1, 0, 1, 0, 1, 0.]),
         rew=np.array([7, 6, 1, 2, 3, 4, 5.]),
     )
-    batch = fn(batch, None, None)
+    batch = fn(batch, buf, 0)
     ans = np.array([7.6, 6, 1.2, 2, 3.4, 4, 5])
     assert abs(batch.returns - ans).sum() <= 1e-5
     batch = Batch(
         done=np.array([0, 1, 0, 1, 0, 0, 1.]),
         rew=np.array([7, 6, 1, 2, 3, 4, 5.]),
     )
-    batch = fn(batch, None, None)
+    batch = fn(batch, buf, 0)
     ans = np.array([7.6, 6, 1.2, 2, 3.45, 4.5, 5])
     assert abs(batch.returns - ans).sum() <= 1e-5
     if __name__ == '__main__':
@@ -66,7 +68,7 @@ def test_fn(size=2560):
         print(f'vanilla: {(time.time() - t) / cnt}')
         t = time.time()
         for _ in range(cnt):
-            policy.process_fn(batch, None, None)
+            policy.process_fn(batch, buf, 0)
         print(f'policy: {(time.time() - t) / cnt}')
 
 
@@ -147,5 +149,5 @@ def test_pg(args=get_args()):
 
 
 if __name__ == '__main__':
-    # test_fn()
+    test_fn()
     test_pg()
diff --git a/tianshou/data/batch.py b/tianshou/data/batch.py
index 301de9a..376e7d2 100644
--- a/tianshou/data/batch.py
+++ b/tianshou/data/batch.py
@@ -73,6 +73,22 @@ class Batch(object):
                 b.__dict__.update(**{k: self.__dict__[k][index]})
         return b
 
+    def __repr__(self):
+        """Return str(self)."""
+        s = self.__class__.__name__ + '(\n'
+        flag = False
+        for k in self.__dict__.keys():
+            if k[0] != '_' and self.__dict__[k] is not None:
+                rpl = '\n' + ' ' * (6 + len(k))
+                obj = str(self.__dict__[k]).replace('\n', rpl)
+                s += f'    {k}: {obj},\n'
+                flag = True
+        if flag:
+            s += ')\n'
+        else:
+            s = self.__class__.__name__ + '()\n'
+        return s
+
     def append(self, batch):
         """Append a :class:`~tianshou.data.Batch` object to current batch."""
         assert isinstance(batch, Batch), 'Only append Batch is allowed!'
diff --git a/tianshou/data/buffer.py b/tianshou/data/buffer.py
index 91fc9c3..1146cd8 100644
--- a/tianshou/data/buffer.py
+++ b/tianshou/data/buffer.py
@@ -39,6 +39,34 @@ class ReplayBuffer(object):
         >>> batch_data, indice = buf.sample(batch_size=4)
         >>> batch_data.obs == buf[indice].obs
         array([ True,  True,  True,  True])
+
+    From version v0.2.2, :class:`~tianshou.data.ReplayBuffer` supports
+    frame_stack sampling, typically for RNN usage:
+    ::
+
+        >>> buf = ReplayBuffer(size=9, stack_num=4)
+        >>> for i in range(16):
+        ...     done = i % 5 == 0
+        ...     buf.add(obs=i, act=i, rew=i, done=done, obs_next=0, info={})
+        >>> print(buf.obs)
+        [ 9. 10. 11. 12. 13. 14. 15. 7. 8.]
+        >>> print(buf.done)
+        [0. 1. 0. 0. 0. 0. 1. 0. 0.]
+        >>> index = np.arange(len(buf))
+        >>> print(buf.get_stack(index, 'obs'))
+        [[ 7.  7.  8.  9.]
+         [ 7.  8.  9. 10.]
+         [11. 11. 11. 11.]
+         [11. 11. 11. 12.]
+         [11. 11. 12. 13.]
+         [11. 12. 13. 14.]
+         [12. 13. 14. 15.]
+         [ 7.  7.  7.  7.]
+         [ 7.  7.  7.  8.]]
+        >>> # here is another way to get the stacked data
+        >>> # (stack only for obs and obs_next)
+        >>> sum(sum(buf.get_stack(index, 'obs') - buf[index].obs))
+        0.0
     """
 
     def __init__(self, size, stack_num=0):
@@ -51,8 +79,26 @@ class ReplayBuffer(object):
         """Return len(self)."""
         return self._size
 
+    def __repr__(self):
+        """Return str(self)."""
+        s = self.__class__.__name__ + '(\n'
+        flag = False
+        for k in self.__dict__.keys():
+            if k[0] != '_' and self.__dict__[k] is not None:
+                rpl = '\n' + ' ' * (6 + len(k))
+                obj = str(self.__dict__[k]).replace('\n', rpl)
+                s += f'    {k}: {obj},\n'
+                flag = True
+        if flag:
+            s += ')\n'
+        else:
+            s = self.__class__.__name__ + '()\n'
+        return s
+
     def _add_to_buffer(self, name, inst):
         if inst is None:
+            if getattr(self, name, None) is None:
+                self.__dict__[name] = None
             return
         if self.__dict__.get(name, None) is None:
             if isinstance(inst, np.ndarray):
@@ -72,13 +118,14 @@ class ReplayBuffer(object):
         i = begin = buffer._index % len(buffer)
         while True:
             self.add(
-                buffer.obs[i], buffer.act[i], buffer.rew[i],
-                buffer.done[i], buffer.obs_next[i], buffer.info[i])
+                buffer.obs[i], buffer.act[i], buffer.rew[i], buffer.done[i],
+                None if buffer.obs_next is None else buffer.obs_next[i],
+                buffer.info[i])
             i = (i + 1) % len(buffer)
             if i == begin:
                 break
 
-    def add(self, obs, act, rew, done, obs_next=0, info={}, weight=None):
+    def add(self, obs, act, rew, done, obs_next=None, info={}, weight=None):
         """Add a batch of data into replay buffer."""
         assert isinstance(info, dict), \
             'You should return a dict in the last argument of env.step().'
@@ -97,7 +144,6 @@ class ReplayBuffer(object):
     def reset(self):
         """Clear all the data in replay buffer."""
         self._index = self._size = 0
-        self.indice = []
 
     def sample(self, batch_size):
         """Get a random sample from buffer with size equal to batch_size. \
@@ -114,16 +160,26 @@ class ReplayBuffer(object):
             ])
         return self[indice], indice
 
-    def _get_stack(self, indice, key):
+    def get_stack(self, indice, key):
+        """Return the stacked result, e.g. [s_{t-3}, s_{t-2}, s_{t-1}, s_t],
+        where s is self.key, t is indice. The stack_num (here equals to 4) is
+        given from buffer initialization procedure.
+        """
         if self.__dict__.get(key, None) is None:
             return None
         if self._stack == 0:
             return self.__dict__[key][indice]
         stack = []
+        # set last frame done to True
+        last_index = (self._index - 1 + self._size) % self._size
+        last_done, self.done[last_index] = self.done[last_index], True
         for i in range(self._stack):
             stack = [self.__dict__[key][indice]] + stack
-            indice = indice - 1 + self.done[indice - 1].astype(np.int)
-            indice[indice == -1] = self._size - 1
+            pre_indice = indice - 1
+            pre_indice[pre_indice == -1] = self._size - 1
+            indice = pre_indice + self.done[pre_indice].astype(np.int)
+            indice[indice == self._size] = 0
+        self.done[last_index] = last_done
         return np.stack(stack, axis=1)
 
     def __getitem__(self, index):
@@ -131,11 +187,11 @@ class ReplayBuffer(object):
         return the stacked obs and obs_next with shape [batch, len, ...].
         """
         return Batch(
-            obs=self._get_stack(index, 'obs'),
+            obs=self.get_stack(index, 'obs'),
             act=self.act[index],
             rew=self.rew[index],
             done=self.done[index],
-            obs_next=self._get_stack(index, 'obs_next'),
+            obs_next=self.get_stack(index, 'obs_next'),
             info=self.info[index]
         )
 
diff --git a/tianshou/data/collector.py b/tianshou/data/collector.py
index ca1bd19..43c5103 100644
--- a/tianshou/data/collector.py
+++ b/tianshou/data/collector.py
@@ -2,10 +2,10 @@ import time
 import torch
 import warnings
 import numpy as np
-from tianshou.env import BaseVectorEnv
-from tianshou.data import Batch, ReplayBuffer, \
-    ListReplayBuffer
+
 from tianshou.utils import MovAvg
+from tianshou.env import BaseVectorEnv
+from tianshou.data import Batch, ReplayBuffer, ListReplayBuffer
 
 
 class Collector(object):
@@ -22,8 +22,8 @@ class Collector(object):
         :class:`~tianshou.data.ReplayBuffer`.
     :param int stat_size: for the moving average of recording speed, defaults
         to 100.
-    :param bool store_obs_next: whether to store the obs_next to replay
-        buffer, defaults to ``True``.
+    :param bool store_obs_next: store the next observation to replay buffer or
+        not, defaults to ``True``.
 
     Example:
     ::
@@ -302,7 +302,7 @@ class Collector(object):
         self._obs = obs_next
         if self._multi_env:
             cur_episode = sum(cur_episode)
-        duration = time.time() - start_time
+        duration = max(time.time() - start_time, 1e-9)
         self.step_speed.add(cur_step / duration)
         self.episode_speed.add(cur_episode / duration)
         self.collect_step += cur_step
diff --git a/tianshou/policy/pg.py b/tianshou/policy/pg.py
index 47b397d..63c05c7 100644
--- a/tianshou/policy/pg.py
+++ b/tianshou/policy/pg.py
@@ -35,6 +35,8 @@ class PGPolicy(BasePolicy):
         discount factor, :math:`\gamma \in [0, 1]`.
         """
         batch.returns = self._vanilla_returns(batch)
+        if getattr(batch, 'obs_next', None) is None:
+            batch.obs_next = buffer[(indice + 1) % len(buffer)].obs
         # batch.returns = self._vectorized_returns(batch)
         return batch