A structural design of a sub-wavelength dual guided-mode resonance (GMR) grating transmission filter was proposed on the basis of the rigorous coupled wave theory (RCWA). The fundamental concept was to use a symmetrical dual GMR grating structure to constrain all electromagnetic energy in the waveguide layer, resulting in efficient resonance peak. In addition, two double GMR grating structures were realized by cascading two single GMR grating structures with or without an air gap. The simulation results show that the filtering effect utilizing the double GMR grating structure without the air layer can be realized at 1 550 nm, whose peak transmittance is about 100% and half-value width (FWHM) can reach to 0.012 nm. The double GMR grating flat-top filter structure with the air layer of 2.13 μm, utilizing the GMR in conjunction with Fabry-Perot resonance (FPR), has a peak transmittance of about 100% at a resonance wavelength of 1 550 nm and a FWHM of 0.15 nm with the flatness of 0.1 dB. The proposed two compact and simple structures show promising potential for optical filtering and sensing applications.