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An IP multimedia subsystem (IMS) has been specified to provide multimedia services in 3GPP release5. In IP multimedia subsystem, both packet-switch domain and IMS authentications are necessary for the IMS subscriber. The IMS authentication is carried out by IMS authentication and key agreement (AKA). It is inefficient that there are so many steps in IMS AKA. Hence, this paper proposes an improved IMS AKA procedure, which can not only guarantee system security but also improve system performance.