We propose a group key agreement protocol in this paper. The key agreement protocol is a good solution to establish a common session key for communication. But in a group of member?s communication, we not only need to establish a common session key, but also need to concern the member changing environment. The proposed protocol is based on weil pairing, ID-based authentication and a complete binary tree architecture. The users in the group will establish a common session key. If there are users want to join or leave the group, our protocol can reconstruct a new common session key for security considerations. Furthermore, our proposed protocol is efficiency when the group member is small and dynamic changing.