The H.264 video coding standard has achieved a significant improvement in coding efficiency over previous standards. However, the computational complexity of the H.264 encoder is increased drastically, which results in implementation for the real time applications practically difficult. Motion estimation is one of the modules in H.264 video encoder that requires significant amount of computation, which occupies up to 70% of total encoding time. This paper presents an effective scheme to reduce the computational complexity of motion estimation to meet real time requirements. The scheme adopts a fast method to predict the initial center of search windows and perform the motion vector search in a halfway stop way, which reduce the computational complexity of motion estimation by exploiting the statistical characteristic of motion vector. The experimental results show that the proposed scheme improves the performance of the H.264 encoder greatly in terms of encoding speed.