In this paper, FWM impacton the downlink channel performance ofdense wavelength division multiplexed gigabit-capable passive optical networks (DWDM-GPONs) has been analyzed. Simulations have been performed on center downlink channels of 7-, 15-, 29- and 35-channel DWDM-GPON systems having 12.5 GHz, 25 GHz, 50 GHz and 100 GHz channel spacings. The downlink channel performance has been evaluated focusing on signal-to-crosstalk ratio (SXR) variations due to FWM impacts. Simulation results obtained for severe FWM impacts give hints about system parameters satisfying a minimum 23 dB SXR, i.e. an appropriate value for system reliability under FWM impacts in current GPON applications.