In order to guarantee higher compression efficiency and video quality

a robust rate-distortion optimization(RDO) is employed in H.264/AVC video coding standard

which is jointly developed by ITUT and MPEG. According to the specification of RDO

all of the prediction modes have to be tried exhaustively in order to find the best mode. As a result

the computational complexity of the mode decision is extremely high and is not suitable for the real time service

such as video conference.This paper proposes a fast intra prediction modes decision algorithm based on the discrepancy within the video picture. In addition

the Direction-Vector based preferred modes selection method is introduced in the proposed algorithm to reduce the candidate modes to be tested. Experimental results are given to demonstrate that our proposed algorithm achieves a reduction of processing time by 40%~50% as compared with the current RDO optimized mode decision with little quality degradation.