A fast adaptive inter prediction modes decision method was proposed to reduce the complexity of H.264 encoder.Firstly the candidate inter modes used in rate distortion optimization can be limited in a small mode group（MG）by using the characteristics of the motion compensated residual image.Then the two most probable modes of the chosen MG are obtained based on the modes of the up MB and the left MB.By calculating and comparing the rate distortion cost of the two modes

the optimum mode of the MB is determined.The experimental results show that the proposed method can save the encoding time up to 65% on average with ?0.24dB performance degradation that can be used in real time applications.