A new scheme for optimization of coarse wavelength-division-multiplexing(CWDM)system based on directly modulated lasers(DMLs)and non-zero dispersion shifted fibers(NZDSF)is proposed.By means of the unique functions(e.g.parameter optimization and iteration sweep)of the optical system design software,the dependence of transmission performance of directly modulated 10Gb/s signals on fiber dispersion is studied.It's proved it's feasible to upgrade CWDM system with the proposed scheme.The results indicate that CWDM network could be implemented cost-effectively by using the negative dispersion fiber and DMLs.