To address the limited computing power of the nodes in Ad Hoc networks, this paper proposed a new session key agreement protocol based on one-way hash chains and mutual authentication technologies. The main contribution of this paper is the achievement of a higher efficiency than other similar protocols. It adopts hash function to avoid the complexity of the traditional public key algorithm, so it noticeably reduces the computational overhead. Meanwhile, this protocol has many secure properties, such as reliability of the node's identity, message integrity, forward security and session key independence. It can also resist the man-in-the-middle attack by using mutual authentication technology.