A macroblock in H.264 has 7 different motion -compensation block sizes in the Inter prediction coding. To achieve as highest coding efficiency as possible

Each mode should be searched and computed in the coding

and rate-distortion （RD） optimization is employed to compute the RDCost. Bus it requires high computational complexity and is very difficult to implement the real-time coding application. In order to reduce the complexity

a fast Inter mode decision algorithm is proposed in this paper

which can select the optimization mode fast through comparing the RDCost. Experimental results show that without considerable performance degradation and bit-rate increase

the coding speeds up greatly with the proposed algorithm.