In Ad Hoc networks it is very difficult to gain the same security as wired networks without the support of infrastructure. To save network?s resources, a group key agreement method using ECDH (Elliptic Curve Diffie- Helleman) based on GDH.3 (Group Diffie-Hellman) protocol is presented, replacing original Diffie-Hellman in GDH.3. Using ECDH in GDH.3 does not change the security performance that?s inherence for GDH.3, but it could save network resource consumption during key agreement. How to implement the improved protocol and the simulation environment are given subsequently. Results from that illustrate the correctness of proposed method.