To provide the differential quality of service (QoS) for different classes of packets and reduce the packet loss probability (PLP), a novel priority-based composite assembly scheme for optical burst switching (OBS) networks is proposed. The low and high packet classes are aggregated into a single burst simultaneously, and the highest-priority packets are placed in the middle, while the low-priority packets are at the tail and head of the burst. The priority is lowered gradually from the middle to the ends. Simulation results demonstrate that the proposed assembly strategy not only guarantees the integrity of the high-priority bursts, but also significantly reduces the average end-to-end delay of the bursts and the PLP of network. So it can adapt to the flexible network with QoS requirement.