Alternating anoxic/oxic conditions have profound effects on both ammonium (AOB) and nitrite (NOB) oxidizing bacteria. In this study the influence of alternating anoxic/oxic condition on nitrifying granules was evaluated in a laboratory-scale column-type sequencing batch reactor (SBR). The anoxic phase was extended from 10 min to 120 min by increasing the anaerobic feeding time. Granules maintained their structure and characteristics during the whole study. The amount of granules with diameter larger than 0.8 mm kept above 95% (mass fraction), and the average settling velocity of particles maintained in the range of 125-130 m x h(-1). Despite the increase in the length of anoxic phases, the values of ammonium removal and nitrite accumulation in effluent still kept at (60 +/- 5)% and (85 +/- 5)%, respectively. Moreover, in the aeration period per cycle, NH4(+) -N removal loading rates, NO2(-) -N and NO3(-) -N accumulation loading rates retained stably at 90 mg x (L x h)(-1), 70 mg x (L x h)(-1) and 15 mg x (L x h)(-1), respectively. All these results suggested the changing anoxic conditions resulted from prolonged anaerobic feeding period had no significant effects on nitrifying granules.