Abstract: We have studied the mixed gas permeation in hollow fiber membrane modules using two approaches: namely, the co- current plug flow model and the complete mixing model with the combination of experimental data. Elucidation was made to determine the permeance of CO2 and CH4 and the selectivity of CO2/CH4 in a polyimide hollow fiber membrane permeator It is found that the intrinsic gas separation properties of hollow fibers for mixed gases can be accurately determined based on (1) the cocurrent plug now model, and (2) the complete mixing model with the assumption of averaged retentate concentration of the feed and the retentate outlet.