IEEE 802.11 WLAN is designed to provide high data speed in small coverage area. 2.5/3G networks, on the other hand, is intended to offer ubiquitous connectivity with relatively low data speed over wide area network. Accordingly, these two technologies can be viewed as a complement to each other. However, for complementary service the two access networks have to be integrated and the service provided by both should be transparent to users. Consequently, one of the main challenges is to provide seamless roaming between them that can meet service requirements. Mobile IP (MoIP) has been considered as the most suitable candidates for achieving seamless roaming. In this paper we investigate interworking architecture between 802.11 WLAN and 2.5/3G CDMA cellular networks with emphasis on the performance of MoIP. A test bed system has been used and results are presented.