Designing efficient and secure group key agreement scheme is one of the key issues in mobile ad hoc networks. In this paper, a novel and feasible group key agreement scheme based on identity is proposed. With this scheme, the direct and secure communications between any subgroups without retransmission could be implemented. The security of the group key agreement scheme can be assured under the complexity assumptions of the ECDLP, CDHP and BIDHP. The detailed analysis of the initialization procedure, subgroup key agreement procedure, group communication procedure and rekeying procedure when membership changes, including joining and leaving, shows that the scheme is efficient in computation and communication cost.