In　order　to　select　the　best　external　carbon　source　for　denitrification,　the　denitrification　characteristics　of　the　commonly　used　external　carbon　sources,　methanol,　ethanol　and　acetate　have　been　studied　using　the　SBR　reactor　and　A/O　nitrogen　removal　process.　Results　showed　that　the　different　external　carbon　sources　had　significantly　influenced　denitrification　activity　and　process.　It　had　been　found　that　the　specific　denitrification　rates　of　methanol,　ethanol　and　acetate　were　reached　up　to　3.2　mg/gh-1,　9.6　mg/gh-1　and　12　mg/gh-1,　respectively.　The　sludge　yield　of　methanol　and　ethanol　was　similar　(about　0.4　g/g),　while　the　sludge　yield　of　acetate　was　very　high　(0.65　g/g).　In　addition,　nitrogen　removal　was　enhanced　immediately　after　ethanol　and　acetate　addition.　However,　methanol　as　carbon　sources　didn＇t　response　immediately　and　had　a　long　adaptation　time.　By　comparison,　ethanol　was　the　best　external　carbon　source,　for　its　high　denitrification　rate,　low　sludge　yield,　quick　response,　easily　obtained　and　low　influence　on　environment.