Modulation instability in decreasing dispersion fibers (DDF) is investigated by means of theoretical analysis and numerical simulation. The gain spectrum of modulation instability is obtained as a function of axially-varying dispersion parameter. The results show that the gain spectrum of DDF is much broader than that of ordinary optical fibers. The broad gain bandwidth can be obtained by selecting the dispersion parameter of DDF. The numerical simulation also shows that fundamental soliton pulses with high repetition rate can be generated in this kind of fiber by modulation instability.