Composite membranes were prepared by the plasma polymerization of hexamethyldisiloxane (HMDS) monomer, and the membranes were applied for the separation of volatile organic compounds (VOC) from air. A tubular type reactor system with an external coil was provided to initiate and maintain glow discharges. Through the plasma polymerization of HMDS, poly(dimethylsiloxane)-like films were prepared. The FT-IR spectra of the plasma-polymerized films were similar to that of poly(dimethylsiloxane) (PDMS). The ratio of discharge power to monomer flow rate (W/F) in plasma polymerization affected the permeability of penetrants. In the plasma-polymerized membranes prepared under low W/F ratio, the permeability was mainly dependent of the condensability of VOCs and the membrane showed the high permselectivity of organic vapors over nitrogen. As the W/F ratio increased, molecular size effect became stronger and the selectivities decreased.