Abstract: The transmission line equivalent circuit method(TLM) is one of the frequently used quick algorithms for shielding effectiveness (SE) prediction of an enclosure. Electromagnetic field modes at the aperture are taken into consideration in the coupling coefficient calculation to improve the accuracy of SE at higher frequencies. Then TLM algorithm is developed to calculate the shielding effectiveness of double metal enclosures with apertures on the common wall. The shielding effectiveness of the inner cavity can be predicted by using the voltage at any position inside the outer cavity to derive the equivalent voltage source at the apertures on the common wall. The results calculated by the proposed method are in good agreement with the simulated results using commercial software CST.