As the number of wireless subscribers rapidly increases guaranteeing the quality of services anytime, anywhere, and by any-media becomes indispensable. These services require various networks (such as CDMA2000 and Wireless LAN) to be integrated into IP-based networks, which further require a seamless vertical handoff to 4^th generation wireless networks. This paper presents a seamless vertical hando. procedure between IEEE802.11 WLAN, which covers hotspot area such as offices, campuses and hotels, and the CDMA2000 cellular network that overlays the WLAN and also covers a larger area. A handoff algorithm between WLAN and CDMA2000 cellular network is proposed. In this algorithm, traffic is classified into real-time and non real-time services. Then, the beginning of handoff is decided by the handoff delay time and throughput according to traffic classes. It is also analyzed mathematically.