Recently, many researchers have investigated the Voice over IP (VoIP) service over Wireless Local Area Networks (WLANs). One ofthe important challenges is to reduce the handoff delay in WLANs. Especially, the channel scan delay, which takes the largest portion of the WLAN handoff delay, is too long to support the delay-sensitive VoIP service. We propose a selective channel scan based on the pre-collected channel information of the neighboring access points (APs) to reduce the channel scan time. Mobile Stations (MSs) collect the channel information of neighboring Access Points (APs) when the VoIP is not used. When the VoIP service is used, MSs perform the selective channel scan mechanism based on the collected channel information. We use the handoff trigger based on the Inter-Arrival Time (IAT) of VoIP packets instead of Received Signal Strength Indication (RSSI). We evaluated the performance of the proposed scheme through computer simulations. The simulation results show that our scheme reduces the handoff delay enough to support the VoIP service over WLANs.