This paper proposes an adaptive p-persistent MAC scheme, named QDA-MAC (QoS differentiation based adaptive MAC scheme), for WLAN to maximize the channel utilization and provide the service differentiation among different traffic stations. Specifically, different from the previous work, the proposed schemedoes not need to estimate the number of active stations for each priority class but still achieves the channelutilization close to its optimal value by exploiting a new parameter, persistent factor, whose optimal value candynamically follow the change of the load based on a simple estimation of the network status. At the same time, thetransmission probability of each priority class can be updated by the optimal persistent factor. Simulation andnumerical results show that QDA-MAC can achieve much higher channel utilization and have shorter delay thanstandard IEEE 802.11 DCF and IEEE 802.11e EDCA in all different WLAN environments.