This paper presents an overlay multicast tree construction algorithm for synchronized realtime media multicast service over Internet which traditionally supports only best-effort service. The proposed tree construction algorithm is implemented to reduce not only the average delay of group members in order to support realtime media service, but also the delay variance among group members to provide an effective synchronized service. Basically, the orthogonal genetic algorithm is employed to obtain the near optimal tree among clusters with low computational complexity since the given problem is NP-complete. In addition, a dynamic tree maintenance process is proposed to effectively update the tree when members change due to leave or join. Finally, we describe experiments that show the superior performance of the proposed algorithm.