The present study attempts to control the micropore size of silica membrane by varying a substituent group in silane precursors. An amorphous silica membrane was produced by thermal decompo- sition of phenylethoxysilane (PTES) on a y-alumina coated α-alumina tube with pore size of 6-8 nm. For the gas molecules of CO ₂, N ₂, CH₄ and C₃H_8, the PTES-derived membraneexhibited higher molecular-sieving property than the membrane formed with tetraethoxysilane (TEOS). Controlling of micropores of silica membrane using phenylethoxysilane as the Si-source was conjectured to result in performance improvements.