For effective CO2 separation using pore size controlled membrane, silica was deposited in the mesopores of a γ-alumina film by chemical vapor deposition of tetraethoxysilane (TEOS) and phenyl-substituted ethoxysilanes at 773-873K. The membranes prepared with phenyl-substituted ethoxysilanes were calcined to remove the phenyl group and control the pore size. The gas permselectivity of prepared membranes was evaluated by using H2, CO2, N2, CH4 and C3H8 single component and a mixture of CO2 and N2. The membranes produced using TEOS contained micropores having permselectivity only to hydrogen, but the phenyl-substituted ethoxysilane derived membranes possessed micropores which are recognizable molecules of CO2, N2 and CH4. In the diphenyldiethoxysilane-derived membrane, the CO2 permeance and selectivity of CO2/CH4 were 10-6 ㎥(STP)·m-2·s-1·kPa-1 and 11, respectively. Therefore, the use of phenyl-substituted ethoxysilane was effective in controlling micropore size for CO2 separation.