The inhibiting form and rule of ammonium to ammonia oxidation bacteria ( AOB) were investigated through a series of batch experiments using biofilm reactor,in which the reaction conditions were controlled to 30 degrees C, HRT 4.76 h, ORP 100 mV. The results showed that when the AOB activity was half-inhibited at an ammonia nitrogen concentration of 2 000 mg x L(-1), the concentration of FA was decreased from 235 mg x L(-1) to 13 mg x L(-1) by reducing pH, leading to the removal of the AOB inhibition. This indicated that FA was the real inhibitor for AOB. The half inhibitory concentration of FA and reaction time required to reach this concentration obtained from the concentration gradient test were fitted. The results confirmed that there was an exponential relationship between them. The inhibitory threshold value of FA to AOB was 98.84 mg x L(-1) under the experimental condition. Both the concentration of FA and the reaction time had a great influence on the inhibition of FA to AOB, and there was a minimal FA concentration that could cause inhibition to AOB. When the concentration of FA was greater than this threshold value, the activity of AOB would be inhibited after a suitable reaction time. High concentration of FA could inhibit AOB within a short time, so could low concentration of FA after an enough long reaction time.