The latest H.264/AVC achieves better coding performance compared to prior video coding standards such as MPEG-2. To provide the interoperability between MPEG-2 and H.264/AVC, the authors propose an efficient inter/intra mode decision for MPEG-2 to H.264/AVC inter P-frame transcoding, which is completed in two stages. In the first stage, a zero-block inter/intra mode decision scheme is employed to select both candidate inter/intra modes for prediction based on MPEG-2 coded macroblock type and motion vector information. Fast motion estimation with various search windows is then incorporated into inter mode decision to improve computation to a great extent. For intra mode decision the authors suggest a sum of absolute transform differences based intra mode decision scheme to further eliminate improbable modes for rate-distortion optimisation calculations. The experimental results reveal that average 85% of computation time (a speed-up factor of seven) can be reduced compared to the fully decoding/encoding procedure. The degradation in the rate-distortion performance is fairly small.