The shielding effectiveness (SE) of a small size perforated shielding enclosure is analysed in this study. For an ideal model, the numerical results by modal method of moment (MoM) match well with the simulation results by a finite integration technique simulator CST-MWS. When adding some detailed structures such as reinforcing rib structures to the ideal model, the modal MoM is not suitable. To verify whether the simulation results by CST on the detailed model are correct or not, the physical enclosure is tested in a microwave anechoic chamber. To measure the SE under the height limitation of the enclosure, an L-shaped antenna is designed and used as the receiving antenna. The measurement results and the simulation results by CST on SE are pretty close.