In this study shielding effectiveness of a rectangular cavity with a rectangular aperture was examined. Electromagnetic shielding is frequently needed to improve the immunity of electronic equipments. The performance of such an enclosure is determined by its shielding effectiveness. Electric and magnetic shielding effectiveness can be calculated as a function of enclosure dimensions, frequency, wall thickness and position within the enclosure. Another important parameter is apertures which negatively affect the shielding problem. Penetration through apertures is undesirable since it lowers the quality of shielding. Electric and magnetic shielding effectiveness were calculated and simulated both at a microwave frequency band an at a specific frequency value.