Starting from the nonlinear Schr??dinger equation, the evolution equations of the perturbation phase and magnitude in optical fibers have been deduced under small signal approximation. Using the obtained solutions of the perturbation phase and power gain, the influence of gain of perturbation propagation is investigated for different initial phases and frequencies. The results show that at the initial stage, the gain is greatly affected by the perturbation initial phase. Depending on the initial phase, all the unstable perturbations have the chance to claim a common maximum gain that is frequency independent. In the normal dispersion region, as well as in the abnormal region where the perturbation frequency is larger than the cut-off frequency and modulation instability is thought not to occur, the perturbation gains oscillate with the propagation distance. In the abnormal dispersion region when the perturbation frequency is smaller than the cut-off frequency, where modulation instability is thought to occur, the gains of perturbations with different initial phases while with the same frequency will evolve to the same positive value through different ways.