To　realize　the　room-temperature　ferromagnetism　(RTFM)　in　diluted　magnetic　semiconductors　(DMS),　we　prepared　a　series　of　Cobalt-doped　ZnO　thin　films　using　pulsed　laser　deposition　(PLD)　at　deposition　temperatures　500　degrees　C　under　oxygen　pressure　from　2.5x10(-4)　Pa　to　15　Pa.　To　elucidate　the　physical　origin　of　RTFM,　Co　2p　spectra　of　cobalt-doped　ZnO　thin　films　was　measured　by　X-ray　photoelectron　spectroscopy　(XPS).　The　magnetic　properties　of　films　were　measured　by　an　alternating　gradient　magnetometer　(AGM),　and　the　electrical　properties　were　detected　by　a　Hall　Effect　instrument　using　the　Van　der　Pauw　method.　XPS　analysis　shows　that　the　Co2+　exists　and　Co　clusters　and　elemental　content　change　greatly　in　samples　under　various　deposition　oxygen　pressures.　Not　only　the　valence　state　and　elemental　content　but　also　the　electrical　and　magnetic　properties　were　changed.　In　the　case　of　oxygen　pressure　10　Pa,　an　improvement　of　saturation　magnetic　moment　about　one　order　of　magnitude　over　other　oxygen　pressure　experiments,　and　the　film　exhibits　ferromagnetism　with　a　curie　temperature　above　room　temperature.　It　was　found　that　the　value　of　carrier　concentration　in　the　Co-doped　ZnO　film　under　oxygen　pressure　10Pa　increases　about　one　order　of　magnitude　than　the　values　of　other　samples　under　different　oxygen　pressure.　Combining　XPS　with　AGM　measurements,　we　found　that　the　ferromagnetic　signals　in　cobalt-doped　ZnO　thin　film　deposited　at　500　degrees　C　under　oxygen　pressure　10　Pa　only　appear　with　the　detectable　Co　2p　spectra　from　incompletely　oxidized　Co　metal　or　Co　cluster.　So　oxygen　pressure　10　Pa　can　be　thought　the　best　condition　to　obtain　room-temperature　dilute　magnetic　semiconductor　about　cobalt-doped　ZnO　thin　films.