The　anatase　and　rutile　TiO　2　film　sensors　were　prepared　by　drop-coating　method　with　calcination　at　450　and　600　°C,　respectively.　After　tested　the　ethylene　sensing　property　over　the　as-prepared　TiO　2　sensors　under　UV　irradiation,　it　was　found　that　the　anatase　TiO　2　as　well　as　the　rutile　sensor　exhibited　a　response　to　ethylene　in　the　dry　nitrogen　atmosphere,　but　with　anatase　showing　a　much　lower　response　to　ethylene　(almost　no　response)　than　rutile　in　the　dry　air　atmosphere　(containing　oxygen).　Moreover,　this　difference　to　the　ethylene　response　between　the　anatase　TiO　2　sensor　and　the　rutile　one　was　also　observed　in　a　wet　air　atmosphere　under　UV　irradiation.　Based　on　the　results　of　photocatalytically　oxidizing　ethylene　over　anatase　and　rutile　TiO　2,　it　is　proposed　that　the　weak　gas　response　to　ethylene　over　the　anatase　TiO　2　under　UV　irradiation　can　be　attributed　to　the　oxidation　of　ethylene　itself.　This　also　suggests　that　a　good　photo-assisted　semi-conductor　sensor　should　have　a　strong　photoresponse　property　but　a　weak　photocatalytic　performance　for　the　sensed　gas.　©　2012　Elsevier　B.V.