TLM method has been applied to calculate the Shielding Effectiveness (SE) of rectangular and cylindrical enclosures with rectangular and circular apertures. To understand the influence of the aperture topology in the SE, the dimensions of aperture are varied and SE of rectangular aperture has been compared with the circular counterpart. The resonant modes inside slotted enclosure have been compared with the enclosure without slot and the behavior has been found similar to that of resonant cavity. This modeling will be helpful in understanding and to mitigate the coupling of EM radiation that is coupled into the enclosures. This is accomplished by identifying the resonant modes of enclosure cavity with apertures. ©2009 IEEE.