Vertical HandOff (VHO) scheme is designed based on the interworking architecture of WLAN/WIMAX which aims to provide always the best Quality of Service (QoS) for users. The performance of applications and network conditions are considered in the handoff process. The handoff procedures are normally initiated when the stations move across the border of WLANs. As a result, both the fixed station and the mobile stations within overlapped areas cannot benefit from VHOs. VHO could be initiated by two factors: mobility when a station moves out of the coverage of its connected WLAN and QoS when the connected network cannot satisfy the requirements.