The gas transport properties of Polyimide membranes were examined as a function of physical aging time. A clear decrease of the gas permeability coefficient was observed. Interpretation of the permeation properties using results from differential scanning calorimeter and dynamic mechanical analyzer indicated that the smaller segmental motion of the polymer chains would be more restrict with aging time. Physical aging reduced the measured solubility coefficients and calculated diffusivity coefficients. Furthermore, the effect of physical aging on selectivity coefficients of polymeric membrane was controlled by the change of diffusivity selectivity. A modified three parameter fit model is obtained and predicts the long-term physical aging behavior with good agreement with experiments.