This paper fist revistits the previously proposed NSAD (New Self-Adapt DCF) mechanism. Some modifications are presented to further enhance the performance of NSAD in the error-prone environment. Then a new NAC mechanism is proposed that can realize bandwidth guarantee by assigning different. self-adapt parameters to users at different priority levels. the bandwidth guarantee property of this new mechanism is analyzed and the highpriority users are found to have bandwidth guaranteed even in heavy contention condition, which is proved true not only by theoretical analysis but also by simulatin results. At the same time the new scheme keeps the self-adapt character of NSAD, so the overall system utilization is kept very high in heavy contention condition compared with the previously studied DCF-based QoS mechanisms.