Ab¬stract – Non-Return-to-Zero (NRZ), Return-to-Zero (RZ) and Dou (duobinary) modulation formats are evaluated numerically with OptSim simulation software at a bit rate of 10 and 40 Gbit/s for single channel and wavelength-division-multiplexing (WDM) systems using long fiber spans and dispersion compensation technique. For the single channel systems, conventional RZ modulation format is found to be optimum. For the WDM systems, especially for 40 Gbit/s, duobinary modulation turns out to be a much better choice in this case since it is more resilient to dispersion, nonlinear-optical-effects (NOE) and is also reasonably simple to implement.