A new video coding standard, H.264/AVC, has recently been standardized. It achieves its high coding efficiency by employing a number of advances in video coding technology. We propose an efficient rate control method for MPEG-2 to H.264/AVC transcoding. Specifically, the values of quantization parameters for I and B frames are computed adaptively based on the side information from the precoded video. Simulations show that our proposed method can meet the target bitrate accurately and achieves a better performance than the rate control scheme of the reference software, H.264/AVC JM 7.4, without increasing the complexity.