In order to guarantee quality of service (QoS) of different priority bursts and effectively reduce the burst loss probability (BLP), an optical buffer mechanism based on priority and burst segmentation in optical burst switching (OBS) networks is proposed. It intercalates the fiber delay units to buffer the segmentation part of bursts in the output data channels. When competition occurs, the header of competition burst data packet or the tail of original burst data packet is segmented according to the priority of bursts. The collision-free parts are exchanged or directly processed in reserved output data channel. Segmentation bursts are buffered in the optical buffer. Simulations show that the mechanism can reduce the burst loss probability of entire networks, and the burst loss probability of the high-priority bursts is much less than those of the low-priority bursts. It also can efficiently reduce the delay and the offset time deficit on QoS guarantee. So the scheme can improve the performance of OBS networks.