|For effective CO_2 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 H_2, CO_2, N_2, CH_4 and C_3H_8 single component and a mixture of CO_2 and N_2. 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 CO_2, N_2 and CH_4. In the diphenyldiethoxysilane-derived membrane, the CO_2 permeance and selectivity of CO_2/CH_4 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 CO_2 separation.