Guided mode resonant filters (GMRFs) for authentication application with low sideband reflection at and azimuthal angles were designed. Using rigorous coupled-wave analysis, the diffractive characteristics of this kind device with different illumination angles, groove depths, and thicknesses of cover SiO2 layer were investigated. The structure of GMRF which satisfies the requirements for authentication applications was obtained. Illuminated at 30\circ for a definite polarization mode, the filter presents symmetrical reflectance shape, low sideband reflectance, two separate reflectance peaks, and definite full-width at half-maximum (FWHM) at 0\circ and 90\circ azimuthal angles.