High performance routers often use crossbar switch. It is important to a crossbar switch to support QoS (Quality of Service). This paper presents a new crossbar scheduling algorithm-OSP(Output Serial Polling) which supports the DiffServ (Differentiated Service) model purposed by IETF. OSP algorithm can deal with cells that have different priority while achieving high crossbar throughput at the same time. Comparing other algorithms, OSP can be implemented more easily in hardware.