Communication network has grown to the stage where it becomes ubiquitous. It allows us to access to on-line services at anytime, anywhere and by any devices. This brings out new services, that was previous only accessible via computers, now are available on mobile devices such as e-commerce applications. These applications require mobile users to be authenticated in order to use the services. In this paper, we proposed a novel authenticated key agreement scheme that allows users and servers mutually authenticate each other. Our scheme also conceals users' identities from adversaries; this is provided in initiator untraceability property of the scheme. Furthermore, the scheme has good computation cost as well as communication and storage costs; thus, the proposed scheme is suitable for the mobile devices.