Abstract: A transmission line/Green function hybrid model to ev aluating the shielding effectiveness of cavity with apertures is presented. The electric field on the aperture is calculated by the transmission line model, wh ich is used as the equivalence magnetic current source to evaluate the field and shielding effectiveness inside the cavity by the dyadic Green function,then th is model is generalized to evaluate the shielding effectiveness of cavity with o ff-center apertures. Numerical results are in good agreement with other metho ds and measured data. The results indicate that the shielding effectiveness at the center of cavity with off-center aperture is better than that wi th center aperture; the larger the aperture is, the worse the shielding effectiv eness willbe; as the aperture length becomes longer, the resonance is broader; th e shielding effectiveness increases when the incidence angle or polarization angle increases.