Polyimide-alumina composite membranes were fabricated by the molecular-wise vapor polymerization (MVP) technique of copolymer which has a constant repeating unit chemically bound by primer on the wall and/or surface of the porous ceramic support. By changing the flow rate of N-2 carrier gas and the repeating number (n), the fabricated pyromellitic dianhydride (PMDA)-diaminodiphenylether (ODA) composite membranes have separation factor alpha(CO2)/(CH4) in the range 0.87-16.0. The estimation of pore diameter was carried out by N-2 permeance and estimated pore diameter was controlled in the range 38-263 nm.