Abstract: This paper investigates the optimum dispersion mapping for 40Gbps WDM system. The paper shows the performance of five channel WDM system with the channel spacing of 100GHz (0.8nm) by using non-zero dispersion shifted fiber (NZDSF) and dispersion compensation fiber (DCF). In addition it reveals both pre-compensation and residual dispersion per span can develop the performance. The modulation format used is RZ with 33% duty cycle. The optimum dispersion mappings for different channel numbers are discussed with different residual dispersions. By applying the optimum dispersion mapping, the impairments in the system can be minimized effectively .The analysis is done with OptiSystem software.