IEEE 802.11e Enhanced Distributed Coordination Function (EDCF) is a new wireless technology in wireless local area networks (WLANs). It defines a new supplement to the existing IEEE 802.11 MAC protocol. In IEEE 802.11e EDCF, the aim is providing a QoS support in WLANs. While the system services different Access categories (ACs), IEEE 802.11e EDCF does not perform well under high load conditions. In order to improve the efficiency, we pay attention to the EDCF of IEEE 802.11e with Block_Ack mechanism. We first proposed a markov chain model and studied the behavior. We extend the model to support IEEE 802.11e EDCF, and presented a more accurate analysis under non-ideal channel environment. We also compared it with that without Block_Ack mechanism under channel error environment.