This paper investigates the quality-of-service performance of multiple decode-and-forward relay system. In particular, we study the outage probability and effective capacity performance of the system based on average channel gain. First, a closed-form expression of outage probability is derived which is more concise than the previous work. Second, we introduce four retransmission strategies and derive the effective capacities of the multi-relay system under these strategies. By means of simulation, we provide the numerical verification of our formula derivation. The simulation results illustrate perfect agreement for our theoretical analysis. In addition, we evaluate the system queue behavior under the strategies. The simulation shows that relay can significantly improve the system queue behavior and that the combined signal processing only pays off under relay-driven retransmission strategies. ; QC 20131212