In this paper, the gas permeabilities of the membranes of copolymer of trimethylsilyl propyne with pentamethyldisilyl propyne (PTMSP-PMDSP) treated by octafluorocyclobutane (OFCB) plasma were studied. The effect of plasma deposition conditions such as deposition time and monmer pressure was studied. At a power of 250 W, OFCB pressure of 5.2 Pa, and deposition time of three minutes, oxygen permeability and oxygen-nitrogen separation factor of the composite membrane were 1020 barrer and 4.18, respectively. The surface composition of the composite membrane was characterized by X-ray photoelectron spectroscopy (XPS). XPS data of plasma modified and unmodified the membranes showed a drastic alteration in the surface structure and a significant decrease in Si/C atomic ratio in the surface layer. It Was not found any relationship between the magnitude of the F/C atomic ratio for these films and their ,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42650246&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42650257&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42650247&type=, ratio.