A method for measuring the change of the refractive index of a loaded body is proposed. The change of the tickness is measured holographicaly, by two double exposure holograms on each side of the body, after delivering an opaque coating onto specimen. On such a way errors, which make occurs during the loading, because of some slope of the specimen, are avoid. A simple plane parallel state of stresses is used for quantitative measurement of the change of the refractive index. Evaluation of the error is made and some results of experiments carried out to approve the method.The possibilities and the future of the method are discused.