Resonant grating filters based on grating abnormality have attracted much attention in recent years owing to their high peak reflectivity，low sideband reflection and controllable line width of central wavelength. Some special characteristics which are hard to be achieved by multilayer stack alternated with high and low refractive indices can be realized with guide-mode resonance of sub-wavelength gratings whose line width of central wavelength can be compressed to less than one nanometer under the condition of weak modulation of gratings. However，the sideband reflections near or far from the central wavelength are still somewhat high due to Fresnel reflection between air and material. According to the effective medium theory (EMT)，sub-wavelength waveguide gratings can be viewed as thin films. In this paper，the sideband reflections of resonant grating filters are effectively reduced through antireflection design based on thin film theory and narrow band resonant filters are attained in the range of visible wavelength with good characteristics.