This paper proposed a novel and efficient scheme to authenticate the H.264/AVC video. The scheme makes an accurate usage of the tree-structured motion compensation, motion estimation and Lagrangian optimization of the standard. Authentication information is embedded strictly based on the best mode decision strategy in the sense that if undergone any spatial and temporal attacks, the scheme can detect the tampering by the sensitive mode change. And the experimental results prove the effectiveness the algorithm against many transcoding and signal processing attacks.