To reduce the complexity of I frame mode decision process,a fast mode decision algorithm is proposedA low pass filter is implemented for each macroblock. After the macroblock is smoothed by this filter we compare the result with the original data and generate the difference between themThen the basic candidate mode class is determined based on the difference,so that the number of candidate modes is reducedTherefore this new method can reduce the complexity of mode decision process in H264 coding at a certain degree with almost no influence on the performance and the bitrate. Further experiments indicate that this new method can save about 16% time in computation with 01dB decline of performance,and less than 1% increase on bitrate