This paper presents a fast mode decision algorithm for H.264 intra prediction based on dominant edge strength (DES). In H.264 intra prediction, the computation-extensive rate distortion optimization (RDO) technique with full intra mode search is used to select the best mode for each macroblock. To reduce the computational load in mode decision, the DES which is corresponding to a decision mode is detected first. In accordance with the detected dominant edge, a subset of the prediction modes is then chosen for RDO calculation. The proposed algorithm only searches 3 modes instead of 9 for the 4×4 luma blocks. As for the 16×16 or 8×8 chroma blocks, instead of 4 modes, only 2 modes are required to be searched. Experimental results revealed that the computation time of the proposed fast intra prediction algorithm is averagely reduced to 30% of the full search method with slight PSNR degradation.