With　domestic　wastewater　of　low　ratio　of　carbon　and　nitrogen　(C/N),　the　control　of　external　carbon　dosage　was　studied　for　the　predenitrification　process　with　continuous　flow.　The　objective　is　to　keep　the　external　carbon　usage　as　low　as　possible　while　nitrate　plus　nitrite　concentrations　(NOx(-)-N)　of　outflow　under　demand.　The　experimental　results　show　that　nitrogen　removal　efficiency　couldn＇t　be　improved　by　increasing　total　recirculating　rate　with　total　recirculating　rate　more　than　2　for　lack　of　carbon　source　and　removal　efficiency　of　TN　couldn＇t　be　largely　improved　by　double　dosage　of　carbon　source　with　constant　recirculating　rate　when　NOx(-)-N　concentration　reaches　about　2　mg/L.　Based　on　the　analysis　of　the　relationship　between　the　external　carbon　addition　and　total　recirculating　rate,　a　control　method　was　proposed.　The　external　carbon　dosage　is　controlled　by　keeping　NOx(-)-N　concentration　of　anoxic　zone　at　the　level　of　2　mg/L,　and　the　total　recirculating　rate　is　determined　by　stated　effluent　NOx(-)-N　concentration.　The　control　method　not　only　can　determine　the　reference　point　of　carbon　dosage,　but　also　can　optimize　the　usage　of　carbon　addition.　It　is　easy　to　be　carried　out　in　wastewater　treatment　plants.