In an overlay wireless network, a mobile user can connect to different radio access networks if it is equipped with appropriate network interfaces. When the mobile user changes its connection between different radio access networks, a vertical handoff occurs. Most existing vertical handoff decision strategies are designed to meet individual needs that may not achieve a good system performance. In this paper, we propose and analyze a novel vertical handoff decision strategy that considers the performance of the whole system. The performance comparison of the proposed vertical handoff decision strategy and existing strategy shows that the proposed strategy is better in the blocking probability of vertical handoff calls.