Voice over IP (VoIP) is a crucial service in the wireless local area network (WLAN) system. The primary medium access control (MAC) technique of the WLAN is IEEE 802.11 distributed coordination function (DCF). Under the situations of various packet sizes, the deficiency of existing MAC protocols mainly comes from the idle slots used to contend the channel for small size packets and from the transmission collisions due to the identical backoff slot value generated for all stations. This study takes account of the above issues and proposes a new MAC scheme to enhance VoIP performance over WLAN. This scheme allows each station to transmit a burst of packets after winning a transmission opportunity instead of just one packet. This reduces the average number of waiting slots and collision probability in each transmission cycle. Moreover, to ensure fairness among stations, a maximum transmission period is assigned to each station in order to control the length of the burst transmission. The simulation results have shown that the proposed method can improve the QoS of VoIP applications significantly by providing lower delay, lower jitter and higher degree of fairness.