In designing integrated optical devices the usual practice is to reduce the problemto 2-D by means of the effective-index method.In this paper we have derived a reducedfotmula foe effective refractive index in Ti:LiNbO3 strip waveguides,calculated thevariations of the profile of effective refractive index with diffusion parameters and discussedthe results,so as to provide a firm basis for the optimal control of the diffusion condition.