An aerobic denitrifier (YS) isolated from swage was mutagenized by UV-irradiation and a high-nitrogen-removal mutant (TB) was obtained. Differences between these two strains in physical and chemical properties and denitrification capacity were studied. The results showed that the denitrification capacity of mutant strains TB was improved. Under the same conditions, the nitrate removal capacity of TB increased from 87% to 93%, and the nitrite concentrations in medium from 212.48 mg x L(-1) reduced to 37.62 mg x L(-1), the removal of nitrite increased rapidly from 15% to 85%, indicating that the strain of TB on the nitrite removal capacity was greatly improved. There was little change in the capacity of the nitrate and nitrite removal after passage, so the genetic stability of TB was better. The effects of different factors on denitrification were investigated and the results were that the optimal culture condition of original strain YS were 1.5% inoculum concentration, pH 6, C/N ratio = 10, and carbon source was glucose; the optimal culture condition of mutant strain TB were 1.5% inoculum concentration, pH 9, C/N ratio = 10, and carbon source was sodium succinate.