The surface of a polybutadiene/polycarbonate composite membrane was modified by ethylenediamine (EA) plasma. It was found that the O-2/N-2 selectivity increased after plasma treatment while the gas permeability decreased, Three phenomena, surface cross-linking, grafting and etching, are often accompanied by plasma treatment, which may affect the membrane permeability and selectivity. It was found that the surface cross linking was responsible for the decrease of permeability and the affinity of EA toward oxygen was responsible for the increase of selectivity. When examining the performance of membranes treated at various plasma powers, surprisingly, it was found that the membrane treated at 70 W, which owned lesser overall EA content than the one treated at 50 W, had a higher O-2/N-2 selectivity and a higher O-2 permeability. The increase of permeability may be due to surface etching. The increase of selectivity was a mystery. After XPS analysis, it was found that although the overall nitrogen content was lower, the surface N/C ratio of the 70 W EA plasma treated membrane was slightly higher than that of the 50 W one. This result may explain why the membrane treated at 70 W possessed a higher O-2/N-2 selectivity than the one treated at 50 W. If this is true, it implies that a high nitrogen-containing thin top layer is all that is required for a high O-2 selectivity.