Integration of communication networks has been accelerated and is recognized as an important evolution plan of the existing networks such as WLAN (Wireless Local Area Network) and WiBro (Wireless Broadband Internet) networks. For the network integration, vertical handoff plays an important role to provide seamless connectivity between different networks. Mobile Node (MN) performs the vertical handoff in the overlapped coverage of these networks. The vertical handoff can be useless if the MN moves at fast speed and penetrates the coverage in a short dwell-time (DT) so that it causes handoff oscillation which is known as a ping-pong effect. We therefore propose a new vertical handoff mechanism to prevent the ping-pong effect in the integrated WLAN and WiBro networks taking into account the MN's velocity. We called our proposed mechanism a mobile-velocity adaptive vertical handoff. The proposed mechanism adaptively performs the vertical handoff according to the DT which denotes the time for MN dwelling in the coverage of the target network, and it is calculated by mobile-velocity and its coordinates. When the estimated DT is larger than the predefined threshold, DTthreshold, the MN performs handoff to WLAN. Since the short DT may cause the unnecessary handoff occurrence, it had better for the MN not to perform handoff and remain it connection in the current network. Simulation results show that the proposed handoff mechanism not only dramatically increases average throughput but also reduces the number of unnecessary handoff occurrence in comparison to typical RSS-based scheme. According to the channel rate setting on access point, our mechanism makes the throughput be differentiated, but the average throughput is improved for the entire network performance. Also the number of unnecessary handoff occurrence is remarkably reduced as the DTthreshold increases.