Abstract: Shielding effectiveness of enclosure with off-center aperture is investigated by using analytical method.Through modeling the shielding and propagation characteristic of enclosure with high order mode equivalent transmission line,the shielding effectiveness of enclosure with aperture is formulated when plane wave is incident with arbitrary incident angle and polarization direction.Relationship between shielding effectiveness and incident angle,polarization direction,aperture size,loss of enclosure wall,test point location and frequency are investigated.The result is verified by enclosure resonant theory.This analytical method with higher speed is suitable for parameter analysis and valuable for shielding enclosure design.