The interworking of the 3G and the WLAN technique provides a perfect connectivity solution in terms of data rate, service cost and area coverage. However the Vertical Handover (VH) from the 3G to WLAN and the Horizontal Handover (HH) between WLAN domains present an additional security challenge. The V/H handover must be fast and secure without impacting the security in both networks. Several authentication methods have been proposed to secure the VH and HH. The Extensible Authentication Protocol Key Agreement (EAP-AKA) is the authentication protocol adopted by the 3rd Generation Partnership Project (3GPP) to authenticate User Equipment by the 3G Home Networks. The EAP-AKA protocol suffers from several weaknesses, such as user identity display and high authentication delay. In this study we propose a new simplify authentication method and key agreement for vertical and horizontal handovers based on the existed method EAP-AKA. Performances analysis of the proposed method show superior results in comparison to the existing EAP-AKA method in terms of bandwidth consumption, signaling cost and authentication delay. The security property of the new method is verified by using the formal security analyzer Automated Validation of Internet Security Protocols and Applications (AVISPA) which has a high talent in finding potential attacks automatically in security protocols.