In designing electronic equipment, Electromagnetic Compatibility (EMC) directives and constraints should be taken into account. In most situations, for an electromagnetic environment, to prevent the devices from the hazardous emissions or to avoid from damages, electronic devices should be housed into shielding metallic enclosures. On the surface of this shielding metallic enclosure, there may be apertures or slots which lead an electromagnetic coupling between the circuits inside the enclosure and outside environment. This apertures and slots could exist for electrical cabling purposes, or cooling purposes, etc., and they are mostly essential. In this research, for loaded and unloaded conditions, calculation of the fields leaking through the aperture inside a resonator with rectangular cross-section are performed using Time Domain (FEM) and hybrid method. In this hybrid method, the Frequency Domain Finite Element Method(FEM) and Moment Method are used in combination. The goal of this combination is to minimize the numerical solution space and process volume.