By complementary integration of WLAN and UMTS, a Beyond third Generation(B3G) mobile network can be constructed to establish the fastest global roaming environments, There are 2 types of B3G networks: loosely-coupled and tightly-coupled, A tightly-coupled network demands lots of investment and considerable amount of time to construct. On the other hand a loosely-coupled network is more scalable and easier to implement than a tightly-coupled one, But it has critical drawbacks of packet loss and blocking of services due to handover delay, To alleviate these drawbacks this thesis proposes a vertical handover procedure between WLAN and UMTS, which is based on MAP of HMIPv6, Performance of the proposal is evaluated in terms of blocking probabilities, The proposed procedure based on HMIPv6 shows better performance than those based on MIP.