In this paper, the stresses and photoelastic effects are calculated in GaAs-GaAIAs multilayer wafers with masked and selective thermal oxidation structure. Authors also propose a new method of Lloyd s mirror interference in a medium for determing directly the refractive index variations caused by the photoelastic effect. The estimated values of index variation from our experiments agree with the theoretical ones within an order of magnitude. The above study would be valuable in the field of semiconductor lasers and integrated optics.