Abstract: A highly efficient denitrifying bacterium aHD7 was screened from activated sludge. After static culture at 30 ℃ for 3 days, the  denitrification rate of the aHD7 reached 91.7%, and during denitrification, nitrite had lower accumulation, with its concentration basically maintained at 1.8 mg·L^-1. The microscopy observation demonstrated that the aHD7 was a gramnegative bacillus, with an average size of 0.5 μm×(1.5-2.5) μm. Based on its biochemical/morphological characteristics and homologic analysis of 16S rDNA sequence, the aHD7 was identified as Pseudomonas mendocina. The investigation on the factors affecting the denitrification capacity of aHD7 showed that at the initial concentration of nitrate nitrogen being less than 276.95 mg·L^-1, the  denitrification rate was almost 100%, and when the initial concentration of nitrate nitrogen was as high as 553.59 mg·L^-1, the denitrification rate could reach 66.8%, with little nitrite accumulated. Ethanol was the most suitable carbon source. C/N ratio 6-8 and pH value 6-9 benefited the denitrification. The aHD7 had a good ability of anaerobic ammonium oxidation, and its average ammonium utilization rate reached 4.56 mg·L^-1·d^-1.