Modulation instability induced by cross-phase modulation in decreasing dispersion fiber(DDF) is investigated.The expression of the dispersion relation is obtained when the fiber loss and the walk-off effect are taken into account.It is found that the gain spectrum of DDF is much broader than that of ordinary optical fibers when the pump power,the propagation distance and the fiber loss are the same with the ones in ordinary optical fibers,respectively.It is also found that using a DDF with a greater dispersion parameter,or using a smaller walk-off effect can broaden the gain spectrum width and increase the maximum value of gain spectrum.The simulation also shows that ultrashort pulses can be easily generated in this kind of fiber by the modulation instability.