The surface of polystyrene membrane treated by Ar, O2 plasma, and the effects were observed before and after the treatment and permeability of CO2, N2 and selectivity of CO2 relative to N2 was measured using continuous flow gas permeation analyzer (GPA). The mole ratio of O over C in the surface was increased from 0 to 0.179 with Ar plasma treatment and route mean square of surface was increased from 15.86 Å to 71.64 Å. Therefore the contact angle was decreased from 89.16° to 18.1°. Thus Plasma treatments made surface of membrane tend to be highly hydrophilic. The optimum condition for the CO2 permeability and ideal selectivity of the plasma treated membrane was as follows: the measurement of Ar (60 W, 2 min, 70 °C) plasma treatment was 1.14×10^-12[m3(STP)·m/m2·sec·atm] and 4.22. In the case of O2 plasma treatment, the contact angle was decreased at 13.56° with increase of O/C ratio (0.189 Å) and route mean square of surface (57.10 Å). The optimum condition for the CO2 permeability and ideal selectivity of the plasma treated membrane was as follows: the measurement of O2 (90W, 2 min, 70°C) plasma treatment was 7.1×10^-12[m3(STP) · m/m2·sec · atm] and 11.5. After plasma treatment, the changes of membrane surface were all subtly linked with both cross-linking and etching effects. Finally, it was confirmed that the gas permeation capacity and selectivity of the modified membrane with plasma could be improved by an appropriate control of the plasma conditions such as treatment time, the power input and sort of plasma gas.