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.