The surface of poly(dimethylsiloxane) membrane was modified by plasma treatments using Ar, N2, O2 and NH3, and the CO2 permeability and selectivity of CO2 over CH4 were measured. The selectivity was found to be remarkably improved by the plasma treatment. The effect of the plasma conditions such as the treatment time, the power input and the gas flow rate on the transport properties of the modified membrane were investigated in detail. The selectivity and the permeation rate of CO2 obtained were 53 and 1.1 x 10(-10) kmol/(cm2.s.Pa) (= 3.3 x 10(-4)cm3/(cm2.s.cmHg)), respectively. The surface of the plasma treated membrane was analyzed by XPS. It was found that the surface structure was fairly different from that of the untreated membrane due to the incorporation of oxygen or nitrogen moieties in the polymer surface. Although the permeation rate of both CO2 and CH4 decreased with the elapse of time after the plasma treatment, this degradation was effectively suppressed by further plasma polymerization of octamethyltrisiloxane on the plasma treated membrane.