An improved resistance model based on Henis-Tripodi’s model for gas permeation in composite membranes has been established, in which the degree of penetration of the coating material into the defects or pores of the effective dense layer has been taken into account. The influences of the degree of penetration, the thickness of the coating layer and surface porosity of the dense layer on gas permeation resistance throughout the composite membrane and separation factor of the composite membrane have been calculated and analysed. Some experimental results have also been explained with this model, The model together with its corresponding coating method is useful for the improvement of membrane separation properties.