In this paper, the effects of an aperture size and polarization type have been investigated by means of measurement results in comparison with simulation. In the investigation, metallic enclosures having different size of apertures were considered that there were 12mm thick wooden blocks covering the inner faces of them. Measurements were conducted in the frequency range of 2.6 GHz to 9 GHz for both vertical and horizontal position of transmitting antenna with respect to longitudinal aperture orientation. It has been observed that the rectangular apertures have better electrical shielding effectiveness (ESE) than square apertures varying from 5 dB up to 28 dB. The ESE of these rectangular apertures which have especially long length and short width has higher values in vertical than horizontal polarization.