The newest video coding standard H.264/AVC provides several predic ting modes and directions for Intra prediction in the spatial region, and the op timal mode was selected by rate-distortion optimization(RDO) with high coding c omplexity. In order to solve this problem, the fast selection algorithm was prop osed from different aspects: Firstly, for luminance mode selection, the method w as proposed that utilizes the image entropy value to predict the mode of the mac roblock, and updates the threshold adaptively by a sliding window. Secondly, int ermediate computing results were reused to reduce the predicting directions. Fin ally, in the RDO performance, the arithmetic structure of Intra prediction was c hanged with calculating the best chromance mode separately. Experimental results s how that with similar distortion performance, the algorithm that adopt the above three strategy can efficiently reduce the computational complexity up to 76% wi th 2.6% increase of bits rate averagely.