Recently, Content Distribution Networks (CDNs) have attracted a great deal of attention from both the industry and academic communities. In this paper, we design efficient object replication algorithms to achieve the optimal performance while not violating clients ’ QoS requirements. We show that such a problem is NP-complete and solve it with a three-stage mechanism: In the first stage, object replication constraints to meet the QoS requirements are generated. Second, a minimal object replication set (MORS), which can satisfy the constraints with the minimal number of replicas on each server, is created. Finally, more objects are replicated on the servers with spare space to further improve the performance. We propose a number of heuristic algorithms and conduct extensive trace-driven simulation experiments to evaluate the performance of these algorithms.