The coherently coupled nonlinear Schrdinger (NLS) equation of the propagation of a light pulse in a fiber has been utilized. We have studied modulation instability of linearly polarized lights with near frequency not only in anomalous dispersion regime but also in normal dispersion regime in polarization-maintaining fibers, when the polarization direction along the two birefringence axes are orthogonal and the intensities along the two birefringence axes are equal. The results show that modulation instability can be produced both in anomalous and normal dispersion regimes. The input pulses have obviously different gain spectra when input power regime is different in normal dispersion regime. Furthermore, the gain spectra are different for different wave lengths (or different frequencies) when the power of input pulse is kept constant.