The couple-wave equations of forward four-wave mixing (FFWM) of ultra-short pulses in single mode optical fibers are given. Numerical investigations using the equations show that, when the walk-off of the pulses is much less than the width of the pulses and the conversion efficiency is lower, the dependence of the FFWM conversion effeciency on the phase mismaching for ultra-short pulses is samilar as in FFWM of continuous waves, there are not any amount ranges of Δk (where k is the mode propagation number un-involving the nonlinear refractive index) wich make the gain equal to zero, and the peak conversion condition should be Δκ=0 (where κ is the mode propagation number which involves the nonlinear refractive index) but not Δk=0. The conversion efficiency is smaller slightly and the peak gain conditions bias a little from Δκ=0 for narrower pulses. Realization of Δk=0 can avoid the broadening and distortion of the pulses to the greatest extent.