In optical burst switching networks, it has been regarded as a convenient and efficient scheme for contention resolution that the contended bursts are buffered in the fiber delay lines (FDLs). Based on the prereservation model, a novel scheme namely delay preemption (DP) is proposed to avoid contention, and the blocking performances of full and partial DP mechanisms are compared. To support the diffserv, the research of DP mechanism is developed in the two-priority system. The blocking model of two-priority system is built up. And the DP mechanisms in terms of blocking and delay are analyzed. The simulation results show that for the high priority bursts, a low burst blocking probability is obtained by using the DP mechanism, which is about two-order level smaller than that of the low priority bursts. Besides, owing to the fixed length of FDL, the requirement of data bursts on delay is guaranteed as well. When the traffic load ρ is 1.0, the average delay time of high priority bursts is confined within 10 μs.