In this thesis, a new relay allocation scheme for the multi-user cooperative communication system with amplify-and-forward (AF) relays is presented. It has formulated an optimization problem for an adaptive relay and power allocation scheme that maximizes system capacity while guaranteeing QoS of each user. Since the optimization problem is no numerical optimal relay allocation algorithm, we propose sub-optimal relay allocation algorithm in this problem. We propose new relay allocation algorithms with two ways. The first scheme we proposed is a simple sub-optimal relay allocation with knowing full channel information. We consider more practical systems because knowing the full channel information is not realistic. The second scheme we proposed is a relay allocation using space-time coding with partial channel information. Our scheme is proper to multi-user cooperative systems with amplify-and-forward relays and has advantages of reducing complexity and increasing capacity as well.