diff --git a/tianshou/data/collector.py b/tianshou/data/collector.py
index 496712e..92a5d99 100644
--- a/tianshou/data/collector.py
+++ b/tianshou/data/collector.py
@@ -75,10 +75,7 @@ class Collector(object):
         self.collect_step = 0
         self.collect_episode = 0
         self.collect_time = 0
-        if buffer is None:
-            self.buffer = ReplayBuffer(100)
-        else:
-            self.buffer = buffer
+        self.buffer = buffer
         self.policy = policy
         self.process_fn = policy.process_fn
         self._multi_env = isinstance(env, BaseVectorEnv)
@@ -92,7 +89,7 @@ class Collector(object):
                     'The number of data buffer does not match the number of ' \
                     'input env.'
                 self._multi_buf = True
-            elif isinstance(self.buffer, ReplayBuffer):
+            elif isinstance(self.buffer, ReplayBuffer) or self.buffer is None:
                 self._cached_buf = [
                     ListReplayBuffer() for _ in range(self.env_num)]
             else:
@@ -118,7 +115,8 @@ class Collector(object):
             for b in self.buffer:
                 b.reset()
         else:
-            self.buffer.reset()
+            if self.buffer is not None:
+                self.buffer.reset()
 
     def get_env_num(self):
         """Return the number of environments the collector has."""
@@ -265,7 +263,8 @@ class Collector(object):
                         cur_step += 1
                     else:
                         warning_count += 1
-                        self.buffer.add(**data)
+                        if self.buffer is not None:
+                            self.buffer.add(**data)
                         cur_step += 1
                     if self._done[i]:
                         if n_step != 0 or np.isscalar(n_episode) or \
@@ -275,7 +274,8 @@ class Collector(object):
                             length_sum += self.length[i]
                             if self._cached_buf:
                                 cur_step += len(self._cached_buf[i])
-                                self.buffer.update(self._cached_buf[i])
+                                if self.buffer is not None:
+                                    self.buffer.update(self._cached_buf[i])
                         self.reward[i], self.length[i] = 0, 0
                         if self._cached_buf:
                             self._cached_buf[i].reset()
@@ -289,9 +289,10 @@ class Collector(object):
                             cur_episode.sum() >= n_episode:
                         break
             else:
-                self.buffer.add(
-                    self._obs, self._act[0], self._rew,
-                    self._done, obs_next, self._info)
+                if self.buffer is not None:
+                    self.buffer.add(
+                        self._obs, self._act[0], self._rew,
+                        self._done, obs_next, self._info)
                 cur_step += 1
                 if self._done:
                     cur_episode += 1
diff --git a/tianshou/policy/imitation/__init__.py b/tianshou/policy/imitation/__init__.py
new file mode 100644
index 0000000..e69de29