AbstractSeveral methods have been proposed for estimating the structural parameters of a waveguide from transmission characteristics of the guided mode. In this paper, a simple and effective method is proposed for estimating the index profile in the cross section of the optical waveguide. In this method the measured field distribution of the guided mode and that calculated from the assumed index profile are compared directly and the index profile is sought for which the two field distributions coincide. To confirm the validity of the method, numerical simulations have been carried out for several graded index slab waveguides. Even if the simplest step index profile is assumed as the initial guess of the profile, the index profile of the waveguide can be estimated with practically sufficient accuracy. In this method, no differential process is needed for the measured values nor a high‐degree smoothing operation. Hence, the method is expected to be superior to others in terms of noise toleranc