AVS (The Advanced Audio Video Coding Standard), established by Audio Video Standard Working Group of China, achieves higher coding efficiency than MPEG-2. In contrast to MPEG-2, where only inter prediction mode with 16x16 block size is used, AVS also supports inter modes with 16x8, 8x16, 8x8 block size. This presents a challenge to MPEG-2-to-AVS transcoder for a good balance between the complexity and performance. In this paper, we propose a fast inter prediction mode selection algorithm including inter-inter and intrainter mode selection. Reusing the inter prediction errors from the MPEG-2 bit stream and intra prediction errors between blocks, we select the best inter prediction mode. The simulation results of many test sequences show that the proposed algorithm reduces the computation cost while maintaining similar PSNR and bit rate. This idea also can be used in other standards to AVS/H.264 transcoding.