Radio interface and wireless service accesses are two areas, in which wireless networks do not provide the same level of protection as wired networks. It may result in the wireless networks being vulnerable to various attacks. For example, the first generation cellular mobile communication systems, such as Advanced Mobile Phone Service (AMPS), and the second generation cellular mobile communication systems, such as Global System for Mobile (GSM), do not take account of security issues comprehensively. It brings about an intruder to be able to eavesdrop user traffic or even change the identities of mobile phones to gain fraudulent services. Therefore, the Universal Mobile Telecommunication System (UMTS), which is known as the third generation (3G) cellular mobile communication system, is proposed. UMTS adopts the security feature of GSM in order to interwork with GSM smoothly. Furthermore, the UMTS adds new security features to design an authentication and key agreement protocol, which is called UMTS AKA protocol. However, it is found that the UMTSAKA has some problems, including bandwidth consumption between a serving network and a user's home network, space overhead of the serving network and sequence number synchronization. Therefore, in this paper, we propose an extension of UMTS AKA protocol, called UMTS X-AKA to overcome the aforementioned problems of UMTS AKA.