Modulation instability of femtosecond optical pulses in decreasing dispersion fibers is investigated. The expression of gain spectrum is obtained. It is found that the broadest spectral width of the gain spectrum can be obtained when the dispersion parameter of the decreasing dispersion fibers fits to a certain equation, and the third order dispersion does not contribute to modulation instability, the range of gain spectrum is narrowed and the growth rate of amplitude is slowed down by self-steepening, and the Raman effect alters the range of modulation instability.