Modulation instability (MI) in single-mode optical fibers is investigated analytically and numerically using a modified nonlinear Schrdinger equation. Due to the role of stimulated Raman scattering, Raman gain spectrum is superposed over the conventional MI gain spectrum in optical fibers. As a result, MI occurs in the normal dispersion region where MI does not appear otherwise; in the anomalous dispersion region, as initial power increases, the spectral range and gain of the conventional MI gain spectrum increase, while the spectral range of the Raman gain spectrum remains unchanged even though its gain increases. Thus the conventional instability region gradually intrudes and eventually screens Raman gain region. Numerical simulations confirms the analytical results and demonstrates that ultrashort pulse trains can be generated by MI in the anomalous dispersion regime, but Raman soliton self-frequency shift appears as the pulse train propagates further.