The　influence　of　oxygen　vacancy　on　the　magnetism　of　Co-doped　ZnO　has　been　investigated　by　the　first-principles　calculations.　It　is　suggested　that　oxygen　vacancy　and　its　location　play　crucial　roles　on　the　magnetic　properties　of　Co-doped　ZnO.　The　exchange　coupling　mechanism　should　account　for　the　magnetism　in　Co-doped　ZnO　with　oxygen　vacancy　and　the　oxygen　vacancy　is　likely　to　be　close　to　the　Co　atom.　The　oxygen　vacancy　(doping　electrons)　might　be　available　for　carrier　mediation　but　is　localized　with　a　certain　length　and　can　strengthen　the　ferromagnetic　exchange　interaction　between　Co　atoms