The H.264 standard achieves much higher coding efficiency than the MPEG-4 standard, due to its improved inter-and intra-prediction modes at the expense of higher computational complexity. Motion estimation is allowed to use seven kinds of block sizes to improve the rate-distortion performance. This new feature has achieved significant coding gain compared to coding a macroblock using fixed block size. However, motion estimation is computational intensive with complexity increasing linearly to the number of allowed block sizes. In order to reduce the complexity, a fast inter-prediction mode decision with macroblock Inner Complexity is proposed in this paper. The analysis results show that the speed improvement of our proposed algorithm over the primitive algorithm is about 10% to 24% times but with little quality degradation.