An experimental method for the refraction index measurement of transparent materials is described,based on the interference of expanded and collimated laser beams that are diffracted by a grating. The interference pattern is captured by a CCD camera,and processed with a home-made software. The optical arrangement is simple and easy to manipulate. The precision of the refraction index measurement is of the order of 10 -4 . Two ways have been provided to carry out the measurements,the second seems to be better in our case, and the angle measurement accuracy is more important than fringe movement positioning. Its application to isotropic polymetric thin film is also presented.