In this paper, a sort-based relay selection algorithm is proposed for decode-and-forward wireless relay systems. The proposed algorithm can reduce computational complexity and system overhead in the relay selection for practical decode-and-forward wireless relay systems with multiple sources and multiple relays. This would be a very important improvement. Firstly, the sufficient and necessary conditions for a relay to be feasible to a source are derived. By adopting relay transmission via its feasible relay, the source can improve channel capacity compared to direct transmission. Then, a sort-based relay selection algorithm is proposed based on the sufficient and necessary conditions. In the proposed algorithm, each relay makes decision on its feasibility individually, but the final source-relay paring decision is made in a centralized manner. Simulation results show that the proposed algorithm can provide considerable system performance improvement over the existing algorithm. Especially at low signal-to-noise (SNR) region, the performance of the proposed algorithm almost reaches the optimal one.