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This paper analyzes the reliability of a two-phase cooperative network. The relays are in decoded-and-forward (DF) mode. Hybrid automatic repeated request (HARQ) is employed in the network to exploit time diversity gain by combining multiple transmissions. In the first phase, the transmission is just traditional HARQ scheme. If one of the relays detects received packets successfully, it forwards regenerated signals in the second phase. When at least two of the relays obtain error-free packets after decoding, two of the relays are selected to forward regenerated signals in Alamouti scheme. The pairwise error probability (PEP) performances of the two phases are analyzed, especially that in the second phase. A closed form expression is given by a specific function. The total packet error rate (PER) is given by defining the decoding function D. The simulation results prove the proposed scheme improves system reliability considerably.