The harmonic and intermodulation distortions induced by fiber dispersions are analyzed. The concept of the common frequency (or common period) is introduced for the analyses of intermodulation. It is shown that both the magnitude and shape of the sum terms of the intermodulation are quite similar to those of the harmonic distortions of the same order. The difference terms of the intermodulation are much smaller if the subcarrier frequencies are not far apart from one another. It is concluded that the influence of dispersion-induced distortions depends on the subcarrier frequency, the dispersion constant, the length of the fiber, and the modulation indexes. Accompanied by the development of high-frequency and large capacity systems and single-mode lasers with extra narrow spectrum, the dispersion-induced distortions may become one of the most important factors in determining information capacity and transmission distance.>