Optical Burst Switching (OBS) is a promising paradigm for the next generation internet. Burst assemble mechanism is one of the key issues in OBS networks. In this paper, a burst assemble mechanism based on burst-length-prediction is proposed. The edge nodes measure the coming packet streams within the pre-defined period for measure, and predict the length of data burst in the fixed assembly time with the results, then transmit the control packets to the core networks ahead the time when the assemble is finished. Theoretical analysis and simulation show that the method can decrease the end-to-end delay of IP packets in optical burst switching network obviously.