Abstract: 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.Resultsshowed 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 3.2mg/g·h^-1, 9.6mg/g·h^-1 and 12mg/g·h^-1, respectively.The sludge yield of methanol and ethanol was similar(about 0.4g/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.