diff --git a/paddle/gserver/layers/CostLayer.h b/paddle/gserver/layers/CostLayer.h
index c2b7803710e82cad9c4f5ba1e6f5042a70d07961..569a6840f0d4432cc827219f590b821df115c7ea 100644
--- a/paddle/gserver/layers/CostLayer.h
+++ b/paddle/gserver/layers/CostLayer.h
@@ -164,8 +164,8 @@ public:
  * tasks.
  * \f[
  * L =
- *   output - label         / -1 < (output - label) < 1 /
- *   sign(output - label)   / otherwise  /
+ *   (output - label)^2 * 0.5  / -1 < (output - label) < 1 /
+ *   (output - label) - 0.5    / otherwise  /
  * \f]
  */
 class SmoothL1CostLayer : public CostLayer {