Composite membranes were prepared by the depoistion of plasmerized coatings onto porous polycarbonates. The monmer chosen for plasma poltmerization was hexafluorobenzene(HFB) which is among flurine compounds well known with good affinity to oxygen. The chemcial structure, surface free engergy and thermal characteristics of pasma-polmerized membence ralated to gas separation properties were investigated in terms of power(W) and feed-in rate of moner(Fm). It was found the structure and charcteristics above-mentioned were affected by discharge power and feed-in rate of monomer. The CF2,CF3 bonds in PP-HFB indicated that the plasma polymerization of HFB was undergone via the repetition of monmer fragmentation and polymeriztion. Also ti was found that PP-HFB had hige hydrophobicity. The oxgen and nitrogen permeation rate(RO2, and RN2)and the selectivity value(RO2/RN2)for the composite membrabes depended on the condition of plasma poltmeriztion and the thickness of the composite membranes depended on the condition of plasma polmerization and the thickness of the PP-HGB. These composite membaranes showed the high owygen permeation rate(RO2=10-5cm3(STP)/cm2seccmhg) and selectivity(RO2/RN2=4) respectively. Tetramethylsilan(TMS) was used to improve the adhesiveness between PC membrance and PP-HFB. As a result, improve the adhesiveness could be improved without decreasing the permsalectivity.