Abstract: An authenticated group key agreement protocol for resource-limited mobile devices was proposed by Tseng Y.M. It is demonstrated that this protocol has security vulnerabilities by mounting a man-in-middle attack against it. The protocol can not achieve key authentication in the presence of an active attacker. This paper presents an improved protocol which achieves mutual authentication. It provides not only the capability of forward secrecy and key authentication, but also the capability against passive attack and man-in-middle attack. The analysis shows that the presented protocol has lower computation cost and communication cost compared with Tseng's protocol.