diff --git a/python/paddle/v2/fluid/layers/nn.py b/python/paddle/v2/fluid/layers/nn.py
index 1c101c62c2dc4c502d0bb5fa3b3835513db5b090..ab443826bd7b4479061b721166c86bfbfc77bc38 100644
--- a/python/paddle/v2/fluid/layers/nn.py
+++ b/python/paddle/v2/fluid/layers/nn.py
@@ -891,6 +891,7 @@ def lstm_unit(x_t,
                 size=4 * size,
                 param_attr=param_attr,
                 bias_attr=bias_attr,
+                act='linear',
                 main_program=main_program,
                 startup_program=startup_program)
     dtype = x_t.dtype