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Identity-based user authentication protocols have been presented to be applicable to resource-constrained devices such as mobile phones. Unfortunately, the previous protocols have the drawback of the key escrow problem. A new protocol of a user authenticated key exchange for the mobile client-server environment is presented based on certificateless public key cryptography (CL-PKC). Our protocol solves the key escrow problem in user authentication schemes based on identity-based public key cryptography (ID-PKC). In addition, the proposed protocol is resisted to both adversaries’ types I and II and achieves perfect forward secrecy. The security of the proposed protocol has been proved using computational Diffie-Hellman (CDH) assumption in the random oracle model. Experimental results show that our scheme is better than He et al. and Tsai et al. schemes respectively in communication cost.