Abstract: Aiming at issues on shielding effectiveness(SE) of a rectangular metallic enclosure with an aperture under electromagnetic radiation, this paper has established a metallic enclosure with an aperture model which is coupling with plane wave by using electromagnetic simulation software CST which is based on finite integration technique(FIT). Some parameters in particular polarization of electric field, materials of enclosure, aperture's size, permittivity and thickness of dielectric which filled in the aperture are concerned in the research on their effect on SE. The works show that: typical metal materials make little difference impact on SE, long side perpendicular to the direction of the eletric field polarization makes big effect on SE, aperture's size can affect the resonant frequency of enclosure, the depth of aperture can affect SE by damping incident wave, an aperture which is filled with dielectric can reduce SE, the thickness and permittivity of the dielectric affects the SE peak point. Results are useful in electromagnetic compatibility (EMC) enclosure design.