With the rapid growth of emerging demand and deployment of wireless LAN (WLAN), much of traffic including multimedia traffic is forced to travel over WLAN. Since the legacy IEEE 802.11 standard is unable to provide adequate quality of service (QoS) for multimedia traffic, IEEE 802.11e group developed medium access protocol (MAC) improvements to support QoS sensitive applications and to make more efficient use of the wireless channel. Although this QoS enhancement is great improvement for the legacy standard in terms of capability and efficiency of the protocol, it does not address the method to enhance QoS when a wireless station (STA) changes its associated access point (AP) due to its mobility. Thus, in this paper, we propose an effective QoS provision method that can guarantee QoS requirements in WLAN when the STA performs handoff and is turned on. In order to guarantee QoS requirements, the proposed method uses APs with dual radio frequency (RF) modules. By adding to AP an RF module which can only receive signals (SNIFFER), the AP can eavesdrop channels of its neighboring APs selected by the modified neighbor graph (NG). Experimental results show that the proposed mechanisms can guarantee QoS for multimedia traffic without ping-pong phenomenon by reducing the link layer (L2) handoff delay drastically and distributing the system loads.