Abstract: A grating structure, sub-wavelength dual-grating structure is presented. This structure is experimentally fabricated using the holographic interference lithography and coating processes. Its resonant spectral characteristics are also tested. Furthermore, the mechanisms and rules of the spectral bandwidth broadening and line-shape improvement of dual-grating and single-grating are studied thoroughly using the vector diffraction theory. Results indicate that the sub-wavelength dual grating is a kind of grating structure with potential for achieving higher security performance and better colorific variable effects. It can be used to broaden its resonant spectra and angular spectral bandwidths and to improve its spectral line-shapes.