In H.264 encoder,all possible coding modes should be checked to choose the most appropriate mode for every macroblock,which adds a heavy computation burden to the encoder.In this paper,a fast inter-mode decision method is presented to reduce computation complexity of an H.264 encoder.By detecting the best matching block(BMB)before transform and quantization,some coding modes can be skipped and the corresponding encoding steps can be omitted for these BMBs.Meanwhile this method can also be used to detect all-zero blocks.The experimental results show that this method achieves consistently significant reduction of encoding time while keeping almost the same rate-distortion performance.