By using the coupled nonlinear Schr; dinger (NLS) equation of the propagation of continuous wave(CW) in a fiber , modulation instability induced by the propagation of CW in normal and anomalous dispersion regime is studied. The results show that the character of modulation instability (MI) depends on the relationship between input power and polarized threshold power , and the maximum value and location of the gain spectra is different when power of input pulse is invariable.