H.264 is the newest video coding standard and has achieved a significant improvement in coding efficiency. The entropy coding methods used in H.264 are CAVLC and CABAC. Although these two variable length code methods can achieve high compression, they are very sensitive to channel errors. This paper presents a joint source-channel MAP (maximum a posteriori probability) decoding method to dealing with this sensitivity to channel errors and applied it to the decoding of the motion vector in H.264 coded video stream. Although H.264 codec has proposed several error resilience methods, we believe this method could provide additional error resilience to H.264 stream. Experiment indicates that our JSCD achieves significant improvement than a separate scheme.