Abstract: Intra-prediction coding and mode decision based on rate-distortion optimization(RDO)technique was used in the H.264. That made compression performance improved,but the complexity and computation load increase drastically.This paper presents an improved and fast algorithms for intra-prediction mode decision based on the feature of Intra_4x4 and Intra_16x16 and the correlation between luminance and chroma. Experimental results show the proposed algorithms increases the speed of intra coding significantly with negligible loss of PNSR and increase of bite-rate.