The　start-up　and　characterization　of　a　three-stage　nitrification　biological　contact　oxidation　reactor　were　studied　using　actual　sewage.　The　experimental　results　showed　that　by　using　the　intermediate　sedimentation　tank　effluent　as　influent　for　the　biofilm　incubation　in　biological　contact　oxidation　reactor,　the　biofilm　matured　within　20　day　without　additional　inoculation　sludge,　and　the　NH4+-N　removal　efficiency　reached　above　98%.　The　biomass　and　biofilm　thickness　of　three　stages　gradually　decreased　in　the　flow　direction,　and　the　highest　biomass　and　biofilm　thickness　were　1271.25　mg/L　and　119.45　μm　respectively.　By　analyzing　the　nitrifying　characteristics,　the　results　revealed　that　the　three　stages　had　higher　specific　nitrification　uptake　rate　(SNUR)　even　at　low　temperature　of　15,　and　the　second　and　third　stages　were　both　higher　than　the　first　stage　at　the　same　temperature　(15,　23,　32).　In　view　of　the　above　phenomenon,　the　ammonia　oxidizing　bacteria　(AOB)　and　nitrite　oxidizing　bacteria　(NOB)　ratio　in　each　stage　were　roughly　estimated　according　to　the　specific　oxygen　uptake　rate　(SOUR).　The　AOB　of　three　stages　were　(25.64±4.89)%,　(34.59±5.02)%　and　(42.50±1.57)%　while　the　NOB　were　(23.52±3.35)%,　(39.65±4.26)%,　(40.69±2.19)%　respectively.　In　addition,　the　fluorescence　in　situ　hybridization　(FISH)　results　derived　on　day　125after　initial　operation　showed　that　the　bacterial　distribution　of　three　stages　do　exist　differences.　The　AOB　and　NOB　were　more　dominant　in　the　second　and　third　stages,　compared　to　the　first　stage.　©,　2015,　Chinese　Society　for　Environmental　Sciences.　All　right　reserved.