Various　kinds　of　vertical　CuO　nanowires　on　the　Cu　substrate　were　prepared　by　thermal　oxidation　method　at　400　degrees　C　in　500　seem　air　with　different　flow　of　water　vapour　(0,　500,　1500,　3000　and　4000　seem).　The　morphology　and　structure　charcateristics　of　as　prepared　CuO　nanowires　were　further　investigated　by　Scanning　Electron　Microscope　(SEM),　Field　Emission　Transmission　Electron　Microscope　(FE-TEM)　and　X-ray　Diffiaction　(XRD),　respectively,　and　their　field　emission　properties　were　also　explored.　The　results　show　that　both　the　density　of　as-obtained　CuO　nanowires　and　their　field　emission　performance　including　turn-on　field　are　greatly　influenced　by　the　water　vapour.　The　CuO　nanowires　prepared　under　the　water　vapour　(500　seem)　exibit　higher　density　and　better　field　emission　performance　than　those　obtained　only　in　dry　air　condition　(500　seem).　The　turn-on　field　of　the　former　is　3.7　V/mu　m,　obviously　lower　than　those　of　the　latter　(6.5　V/mu　m).　Additionally,　the　field　emission　performance　of　as　prepared　CuO　nanowires　improves　with　the　increase　of　water　vapour　flow　in　the　preparation　at　first,　reaches　the　extreme　value　at　3000　seem,　and　then　turns　down.　The　best　field　emission　performance　of　CuO　nanowires　is　obtained　in　3000　seem　water　vapour　where　the　lowest　turn-on　field　is　about　1.4　V/mu　m.