Nanometric diffraction grating structure exhibits the same macroscopic properties as the effective layer of uniaxial continuous material. Power reflectance of such dielectric lamellar structure was computed using RCWA theory. Angular spectra of power reflectance were subsequently used as the input to the optimization procedure in order to find effective indices of refraction. The spectral dependence of effective parameters was obtained as well as the fill factor dependence. The results were confronted with the simple model of effective permitivitties. The results support the idea, that the effective media models should include the role of the substrate.