Density　functional　theory　(DFT)　calculations　are　employed　to　investigate　the　reactivity　of　tungsten　oxide　clusters　towards　carbon　monoxide.　Extensive　structural　searches　show　that　all　the　ground-state　structures　of　(WO3)n+　(n　=　1-4)　contain　an　oxygen　radical　center　with　a　lengthened　W-O　bond　which　is　highly　active　in　the　oxidation　of　carbon　monoxide.　Energy　profiles　are　calculated　to　determine　the　reaction　mechanisms　and　evaluate　the　effect　of　cluster　sizes.　The　monomer　WO3+　has　the　highest　reactivity　among　the　stoichiometric　clusters　of　different　sizes　(WO3)n+　(n　=　1-4).　The　reaction　mechanisms　for　CO　with　mono-nuclear　stoichiometric　tungsten　oxide　clusters　with　different　charges　(WO3-/0/+)　are　also　studied　to　clarify　the　influence　of　charge　states.　Our　calculated　results　show　that　the　ability　to　oxidize　CO　gets　weaker　from　WO3+　to　WO3-　as　the　negative　charge　accumulates　progressively.　This　journal　is　©　the　Owner　Societies　2015.