Abstract: A rigorous numerical method for the prediction of shielding effectiveness, which adopts the mode-matching technique and the mixed potential integral equation based on the method of moments, has been presented for rectangular metallic enclosure with aperture. Some influence factors, such as aperture thickness, aperture shape, incident polarization direction and high-order modes, are considered. By the numerical simulation of classical models, the high accuracy of this method is verified, and the efficiency of the method is remarkable compared with other numerical algorithms. Furthermore, some valuable conclusions in engineering are obtained: When the polarized direction of the incident wave is parallel to the short side of the rectangular aperture, the coupled energy will attain the maximum