In this paper, we consider an environment where a number of users are geographically bounded and analyze a performance of multicast transmission via a relay station. Generally, common data rate of a conventional multicast system is determined by a minimum link capacity between a base station and multiple users. However, common data rate of the multicast system based on relay station is determined by not only a link capacity between the base station and relay station but also a minimum link capacity between the relay station and multiple users. Considering these characteristics, we derive the number of users and a range of received signal power of relay station to show that the relay station based multicast system guarantees more quality of service than conventional multicast system.