The integration of the 3G networks and Wireless Local Area Network (WLAN) has drawn much attention from both industry and academics in the past few years. 3G cellular networks provide worldwide coverage, while WLAN can be deployed to get high bandwidth internet access in hotspots. These two kinds of wireless networks are complementary and should be integrated to make sure that, subscribers can always be best connected to the networks. Vertical handoff, which maintains the connection active during switching between different access network such as a UMTS and WLAN, is a critical problem for the integration of these two wireless technologies. The technique used for vertical handoff is much more complex than that of the corresponding horizontal handoff used in homogenous networks. In this thesis we have proposed a new procedure for handoff that through the decreasing of utilization correspondent node and home agent in the movement of mobile node, reduces the number of signaling and registration processes. We have also decreased energy take up of the system via estimating user's locations and through that decreasing the on time for interfaces. Using this procedure handoff delays and packet loses in the handoff process were declined increasing throughput of the integrated network. Disconnections caused by existence of black holes in WLAN network were also decreased using the proposed procedure. We have also simulated handoff delays and networking throughputs in this thesis, using NS-2 simulation.