The　photo-responded　behavior　of　ZnO　and　its　sensing　response　to　ethylene　or　acetone　under　visible-light　irradiation　at　room　temperature　were　reported.　A　ZnO　sensor　without　any　response　to　target　gas　in　the　air　atmosphere　in　dark　at　room　temperature,　exhibits　not　only　a　photo　response　but　also　an　obvious　sensing　response　to　target　gas　in　the　air　atmosphere　under　visible-light　irradiation.　It　is　suggested　that　the　non-intrinsic　photo-absorption　induced　by　the　native　defects　of　ZnO　(i.e.,　the　two-photon　or　multi-photon　excitation　process　via　the　intraband)　may　be　responsible　for　the　photo-responded　behavior　to　the　visible　light.　Moreover,　a　mechanism　model　that　accounts　for　the　solid-state　process　of　photo-carrier　generation/recombination　is　proposed　to　explain　the　photo-induced　gas　sensing　response　of　ZnO　under　visible-light　irradiation.　This　indicates　that　the　photo-activated　gas　sensing　response　property　of　semi-conductor　can　be　also　caused　by　its　non-intrinsic　photo-absorption.　©　2013　Elsevier　B.V.