Abstract: Wireless ad hoc networks are a kind of networks that do not need communication infrastructure. Such networks are being used in more and more areas. The security of ad hoc networks has become a hot research point in the latest years. The security of cluster based ad hoc networks is analyzed and a new scheme is proposed to solve the security problems existing in such networks. The main contributions of the scheme are as follows: ① presenting a security mechanism to secure the cluster heads; and ② proposing a mechanism to remove the security defects in the roaming of mobile nodes, and providing a related algorithm. Simulation is performed to test the scheme. The experiment results show that the scheme is feasible. Compared with related security schemes for cluster based ad hoc networks, the proposed scheme is more secure, and has a better network performance.