Phosphorus and nitrogen are the important eutrophication nutrients. They are removed in the anoxic/oxic reactor through simultaneous precipitation and biological nitrogen removal. The effect of alum a commonly used simultaneous precipitant on biological nitrification and denitrification are investigated in the present study. Simultaneous removal of phosphorus was carried out using the coagulant alum Al2(SO4)3·14H2O at 2.2 mol ratio. Before the start of simultaneous precipitation the nitrification rate of the A/O reactor was found to be 0.05 g N-NH4+/g VSS/d. It starts to decrease with increase in coagulant dosage. The nitrification rate for alum dosage 97.13 mg/L was 0.38 g N- NH4+/g VSS/d. There was no accumulation of nitrate in anoxic tank. The nitrogen removal efficiency of the reactor was affected and it fell from 88 to 78%. There was a slight decrease in effluent COD from 16∼20 mg/L to 8∼12 mg/L after the introduction of simultaneous precipitation into the reactor. The usage of alum as a simultaneous precipitant in the anoxic/oxic reactor was limited due to its inhibition on nitrification. Alum did not have any affect over denitrification process.