This paper presents a novel rate control initialization algorithm for H.264. Firstly the first instantaneous decoding refresh frame in a video sequence is intra-coded to get the mean absolute difference (MAD) value, then the average MAD value of the second, third and fourth frames which are inter-coded is computed, finally the initial quantization parameter is determined depending on the bits per pixel and the two MAD values. When compared with the algorithm JVT-G012 (2003) which is the current recommended algorithm implemented in the H.264 reference software JM13.0 and the algorithm by H.Wang and S.Kwong (see IEEE Trans. Circuits Syst. Video Techno!., vol.18, no.1, p.140-4, Jan. 2008), the proposed algorithm can increase the average peak signal-to-noise ratio of reconstructed video by up to 1.241 dB and 1.262 dB, respectively.