The effects of UV-irradiation on the properties of Al₂O₃ gel films obtained from Al(O-sec-Bu)₃ chemically modified with benzoylacetone have been studied. These gel films showed an optical absorption band at around 325nm characteristic of the π-π* transition in the chelate ring. The irradiation of UV-light of 365nm on the gel films dissociated the chelate ring and simultaneously decreased the solubility of the gel films in acetone solutions containing HNO₃. This finding was successfully applied to the fine patterning of Al₂O₃ films. The gel films were irradiated with UV-light and then leached in HNO₃-acetone solutions, followed by the heat treatment at 400°C to form finely patterned Al₂O₃ films on a variety of substrates.