This study was primarily to find the separation characteristics of CH₄-CO₂ mixed gas, using polyimide hollow fiber membrane. For the purpose, the experiments on the sorption-permeation charcteristics of CO₂, CH₄ pure gases and mixed ones respectively were made. The experimental conditions were as follows : feed flow rate 0-15 l/min, system temperature 30-70℃, and applied pressure 0-20 bar. The membrane materials used in the experiments were a kind of polyimide, which is glassy polymer. The pure gases permeability of CO₂ and CH₄ could be fully explained by the partial immobilized dual-mode sorption model, and in case of mixed gases, some deviation by the nonidality of CO₂ gas was exposed. Idal separation factor(α^*=P_CO2/P_CH4), the permeability ratio of the two gases, was a little decreased according to the increase of feed pressure, while in much larger scale decreased with the increase of system temperature, and when the deviation to the nonideality was observed, with fugacity instead of pressure as permeation driving force, the difference was found within 5% in the range of this experimental condition(0-20 bar).