The security issues in ad hoc network are increasingly important. In this paper, we propose a distributed key management what approach by using braid groups and key tree. Without any assumption of prefixed trust relationship between nodes, the proposed method works in a self-organizing way to provide the key management services. The using of the proposed tree-based braid groups has following advantages: (1) the communication cost is minimized to constant time; (2) the complexity of computation is decreased to linear permutation by avoiding modular exponential operation. Our approach is more simple, secure and efficiency for group key management in MANET. Secure group communication requires scalable and efficient group membership management with appropriate access control measures to protect data and to cope with potential compromises. To this end, a secret key for data encryption must be distributed securely and efficiently to current members. The group key must be changed to ensure backward and forward secrecy when membership topology changed. Several proposals for group key management have been made recently. They range from key distribution schemes for large-scale single-sender multicast to contributory key agreement schemes for small any-to-any peer groups. Although most of them focus on wired networks, extensions to wireless networks including MANET should be explored as such networks are becoming more common place. Due to the lack of fixed infrastructure and limited resources, it will be much more complex to adapt protocols and other technologies from the infrastructure based networks. 1.1 Our Contribution The purpose of our research is to design an efficient key agreement protocol for a group communication in mobile ad hoc networks environment. Our protocol only secure against a powerful passive adversary who can intercept any broadcast message over public channel without authentication. There are two techniques to be implemented in our protocol including braid groups cryptographic and key tree. The braid groups cryptographic is used to decrease computation cost and key tree is used to reduce communication cost. The protocol is designed to be contributory key agreement without trusted third party or permanent controller. The members generate the shared group key in contributory manner. Furthermore, the radio signal strength is applied to reduce communication time. The nearest node between existing group members and a new member is leader of the group at that moment. The message can be fastest transferred to each other. Moreover our protocol, the modified STR using braid group support dynamic membership group operations including join, leave, merge, partition and key refreshing to satisfy security requirement including group key secrecy, forward secrecy, backward secrecy and key independence. 1.2 Related Works