The formation and long-term stability of nitrifying granules in a sequencing batch reactor was investigated in this study. The results showed that nitrifying granules with a size of 240 mu m and SVI of 40 ml g(-1) were formed on day 21 at a settling time of 10 min. Maintaining settling time at 15 min from day 57 to 183 did not affect the physical characteristics of sludge and the fraction of suspended floc in the sludge. In addition, nitrifying granules could tolerate the fluctuations of nitrogen loading rate from 0.72 to 1.8 gI(-1) d(-1) during 2 months without the change of physical characteristics. However, it was observed that complete nitrification to nitrate and partial nitrification to nitrite by sludge converted each other corresponding to the change of the influent NH4+-N concentration. Thus, an appro priate method is needed to maintain a stable complete nitrification or partial nitrification under the conditions with changing influent NH4+-N concentrations and nitrogen loading rates. (C) 2007 Elsevier Ltd. All rights reserved.