The one dimensional wave equation was transformed into space-frequency domain by Fourier transform, and became an integral equation which could be solved by Nystrm method. A solver of standard matrix eigenvalue problem from the integral equation was given by Lapack packages. The dispersion relation and mode fields of a planar dielectric waveguide could be obtained simultaneously. The numerical results showed that the proposed method was flexible for planar optical waveguides with any refractive index profiles and high accuracy only using a low order matrix.