We investigate a distributed beamforming for decode-and-forward (DF) cooperative protocol. We show that the distributed channel co-phasing (DCC) is optimal and the received SNR of DF DCC is shown to be always greater than or equal to that of ideal distributed space-time block coded (D-STBC) protocol. We address an approximated expression for the probability density function (PDF) of the received SNR for DF DCC under the Rayleigh fading channels, with which the BER is analytically provided. We also investigate the robustness of DF DCC on phase error caused by time delay and frequency separation of reciprocity channel. Numerical investigation shows that the DF DCC is robust for the phase error caused by time delay τ. Moreover, DF DCC with frequency separation χ = 1MHz still shows the performance gain against the ideal D-STBC under τ ≤ 8 ∗ 115.2µsec.