Abstract: A heterotrophic nitrobacterium strain JZ1-1 with higher nitrifying capability was isolated and mutagenized from an acclimated activated sludge. The JZ1-1 was identified as Colloidessp., according to its morphological and physiological features. The factors affecting the nitrifying capability of JZ1-1 were investigated, including medium carbon source, C/N ratio, pH value, dissolved oxygen, temperature, and ammonium nitrogen concentration. The optimal carbon source was sodium citrate, and the nitrification was favored when the C/N ratio was from 10 to 14, temperature was 30 ℃, and pH value was 6-9 when cultured at a rotating speed of more than 150 r·min^-1. JZ1-1 could degrade ammonium nitrogen effectively when the initial concentration of ammonium nitrogen was from 100 mg·L^-1  to 500 mg·L^-1. JZ1-1 was stable after 5 generations of subculture.