In this study, the conversion efficiency and optimal conditions of successive nitrification and denitrification were assessed for Nitrosomonas europaea, which uses ammonia monooxygenase (AMO), nitrite reductase and nitrous oxide reductase. In aerobic conditions, the first step of nitrification (i.e. NH4+ --> NO2-) occurred; under oxygen-limiting or oxygen free-conditions, however, there was denitrification, whereby nitrous oxide and dinitrogen were produced from nitrite (NO2-). About 7% of the total ammonium was converted to dinitrogen following the successive nitrification and denitrification by N. europaea. During nitrification and denitrification, the optimal pH range for the production of nitrite and dinitrogen was found to be 7.0-8.0. A low partial oxygen pressure or oxygen-free conditions were favorable for the production of dinitrogen.