In the work the effect of orientational inhomogeneity of nematic liquid crystals (NLC) on characteristics of diffraction optical elements (DOE), based on the system relief grating-liquid crystal, was considered. The analytical solution for the NLC director and refractive index spatial distribution has been obtained and the diffraction efficiency of the system relief grating-liquid crystal has been calculated for different diffraction orders. As a result, it has been found that inhomogeneous orientation of NLC leads to considerable variation of the diffraction efficiency across the DOE area.