The coherently coupled nonlinear Schrodinger (NLS) equation of the propagation of a light pulse in a fiber has been utilized. We have studied modulation instability of linearly polarized light not only in anomalous dispersion regime but also in normal dispersion regime when the intensities along the two birefringence axes are equal. The results show that the input pulses have different gain spectra obviously when input power regime is different, both in anomalous dispersion regime and in normal dispersion regime. Furthermore, the gain spectra is different for different intrinsic birefringence when power of input pulse is invariable.