It is essential to design QoS supported MAC mechanism for supporting QoS in WLAN. In [1], we have proposed a new backkoff algorithm, named RWBO+BEB, to decrease the packet collision probability significantly. In this paper, we explore how to make RWBO+BEB support service differentiation in WLAN, and propose a novel proportional service differentiation algorithm, named p-RWBO, to allocate the wireless bandwidth according to the bandwidth ratio of each station. The basic idea of p-RWBO is that different priority stations use different walking probability, p w , which is a key parameter in RWBO+BEB. An analytical model is proposed to analyze how to choose p w according to the bandwidth ratios of stations. The simulation results indicate that p-RWBO can allocate the wireless bandwidth according to the bandwidth ratio of each station.