The characteristics of Fermi index profile have been analysed. It is shown that the quasi-step-graded index profile can be approximated by Fermi index profile. The equations of modes for planar optical waveguides with Fermi index profile have been obtained. The data calculation and the comparison between theory and experimental results show that the characteristics of dispersion for optical waveguides with Fermi index profilecan be excellently determined. The accuracy is better than the approximation of Step index profile.