A fast algorithm based on direction in intra frame downsizing in H.264 is proposed, which used modes information of macroblocks before transcoding and the direction relation of modes between decoding and re-encoding in transcoding. This algorithm also made use of statistics between decoded modes and re-encoded modes, which came from a lot of sequences data experiments. Without full modes encoding, it can improve the speed of reducing intra-prediction frame resolution obviously. Comparing to traditional transcoding, it only needs to compute one of thirteen modes in re-encoding. The experiments show that this algorithm can significantly speed up 92 percent transcoding time in intra-prediction frame of H.264 with slight PSNR degradation. It also can support an improvement in real-time for transcoding and ability of bandwidths changing.