The increasingly extensive applications of the latest video coding standard H.264 creates an urgent need to transcode the widely available MPEG-2 compressed video to H.264 compressed format. This paper presents a novel fast variable block-size selection algorithm for inter macro block (MB) based on the MB motion information aiming for providing low-complexity transcoding technique. The edge pattern of the MB residual as well as motion vector is employed for analyzing the MB motion characteristics. An optimal block size is decided based on the characteristics without conducting motion reestimation. Experiment results show that this method reduces the transcoder complexity dramatically with little compression performance degradation.