In H.264/AVC

intraprediction is used to improve the coding efficiency of I frame. There are one size of 8×8 for the chroma block and two sizes including 4×4 and 16×16 for the luma block. Each kind of block provides many prediction modes

so that this multimode selection increases the coding complexity greatly while improving coding efficiency. In this paper

a fast intraprediction mode selection algorithm with different criterion based on RDO(Rate distortion optimization) is proposed using the mode selection relation of chroma block and luma block and the relation of luma 4×4 block and luma 16×16 block. Experiment results show that our proposed algorithm can averagely speed up to times while maintaining similar PSNR and bit rate.