Abstract: An alumina film(AAO/Al) was prepared on Al plate surface by anodic oxidation. The photodecompositions of oleic acid and C2H4 were examined on the AAO/Al and a nude Al piece with FTIR-ATR and a batch reactor, respectively. The function of AAO film in the photodecomposition was discussed by combination of the activity studies with the results of characterizations of UV-DRS and FTIR. It was showed that the AAO/Al could absorb UV light of wavelength ＜ 265 nm, and the nude Al piece didn't absorb the light of wavelength ＞ 220 nm. The AAO/Al could enhance the photodecomposition of oleic acid and ethylene by irradiation of 254 nm UV light but the nude Al piece couldn't. For on AAO/Al, the conversions of oleic acid and ethylene were 70% (upon the illumination for 3 h) and 9% (upon the illumination for 4 h ), respectively；while for on the nude Al piece, the conversion of oleic acid was 30% and no oxidation of ethylene occurred. AAO/Al didn't present any enhancement to the photodecomposition under irradiation of 365 nm light, the conversion of oleic acid was ca 20% which was comparable with that on nude Al, and the photodecomposition of ethylene didn't occur on both samples. It was suggested that the AAO film could be excited by 254 nm light so as to present photocatalytic function, which resulted from the superfluous Al atoms and oxygen-deficient defects in its structure.