We　have　studied　the　reaction　mechanism　of　CO　oxidation　on　the　Cu-13　cluster　via　density　functional　theory.　There　are　two　main　reaction　pathways　to　be　considered:　Eley-Rideal　(ER)　and　Langmuir-Hinshelwood　(LH)　mechanisms,　respectively.　According　to　these　two　main　reaction　mechanisms,　we　have　obtained　five　reaction　pathways　for　the　first　CO　oxidation　(denoted　as　R-ER1,　R-ER2,R-　R-LH1,　R-LH2　and　R-LH3,　respectively):　R-ER1　is　CO(gas)　+　O-2(ads)　-＞　O-(ads)　+　CO2(gas);　R-ER2　is　CO(gas)　+　O-2(ads)　-＞　CO3(ads)　-＞　O-(ads)　+　CO2(gas);　R-LH1　refers　to　CO(ads)　+　O-2(ads)　-＞　O-(ads)　+　CO2(gas);　R-LH2　refers　to　CO(ads)　+　O-2(ads)　-＞　OOCO(ads)　-＞　O-(ads)　+　CO2(gas)　and　R-LH3　refers　to　O-2(ads)　+　CO(ads)　-＞　O-(ads)　+　O-(ads)　-＞　CO(ads)　+　O(ads)　+　CO2(gas).　These　pathways　have　low　energy　barriers　and　are　strongly　exothermic,　suggesting　the　Cu-13　cluster　is　very　favorable　catalyst　for　the　first　CO　oxidation.　However,　there　are　higher　energy　barriers　of　99.　8　and　45.4　kJ/mol　in　the　process　of　producing　and　decomposing　intermediates　along　the　R-LH2　and　R-ER2,　indicating　that　RERi,　Run　and　R-LH3　are　superior　pathways　with　lower　energy　barriers,　especially　the　R-ER1　channel.　Thereafter,　the　second　CO　is　more　prone　to　react　with　the　remaining　oxygen　atom　on　Cu-13　along　the　ER　channel　in　comparison　with　the　LH　pathway,　in　which　the　moderate　barrier　is　70.0　kJ/mol　and　it　is　exothermic　by　59.6　kJ/mol.　Furthermore,　the　interaction　between　the　absorbate　and　cluster　is　analyzed　by　electronic　analysis　to　gain　insights　into　high　activity　of　the　copper　cluster.