IEEE 802.11e standard is proposed to support Quality of Service (QoS) in wireless local area networks (WLANs). The distributed access method of 802.11e, Enhanced Distributed Channel Access (EDCA), prioritizes traffic generated by delay-sensitive applications by increasing probability to access medium for this type of traffic. EDCA resets the Contention Window (CW) of mobile stations to a minimum value after each successful transmission. This static behavior is not adapted to dynamic conditions of wireless network as it degrades network performance when channel access requests increases. Several mechanisms focusing on network adaptation have been proposed to improve the performance of IEEE 802.11e standard. The main limitation for these approaches is slow adaptation to the network state when the channel experiences bursty traffic.In this paper, we propose a new dynamic approach for contention window adjustement for each Access Category (AC) according to the traffic and network conditions. We compare our proposal with the IEEE 802.11e EDCA standard as well as some other mechanisms in the literature.Simulation results show that our proposed approach outperforms EDCA of 802.11e as well as some other enhancement to the standard present in the literature.