Four-wave mixing (FWM) is an important nonlinear phenomenon limiting the performance of optical fiber networks. In this paper, a comparative analysis of the FWM impact on the network performance of long-haul dense/ultra-dense wavelength division multiplexing (DWDM/UDWDM) optical communication systems using standard single mode fibers (SSMFs) and non-zero dispersion shifted fibers (NZDSF) has been presented. Simulations have been perlbrmed on central channels, the most intensively affected channels by FWM among all, of 15-, 35- and 63-channel systems having channel spacings of 3.125 GHz, 6.25 GHz, 12.5 GHz and 25 GHz. Network perlbrmances have been evaluated and compared by lbcusing on signal-to-crosstalk ratio (SXR) variations due to FWM impacts. Simulation results show worst-case values and thereibre they are important in choosing system parameters satisfying a minimum 23 dB SXR, which is an appropriate value for current network applications showing system reliability under FWM impacts.