A feasibility of a membrane-aerated biofilm reactor (MABR) for controllable nitrification was examined. The estimation of oxygen supply rate (OSR) with three polyacrylonitrile membrane modules revealed that specific OSR was equivalent in these membrane modules and OSR affected only air pressure, thus enabling control of aeration simply by adjustment of air pressure. A continuous nitrification experiment consisting of three reactors differing in membrane surface area investigated the reactor performance of the MABR at an air pressure of 23 kPa. The results indicated that the ammonia removal rate at steady state was dependent on membrane surface area, at rates nearly equivalent to that predicted by the above OSR experiment. The amount of bacteria adhering to the membrane surface was not completely proportional to membrane surface area due to clogging in a reactor with high membrane surface area, which accompanies a decrease in specific ammonia removal rate per biomass with membrane surface area. Stable ammonia removal rates at air pressures of 23, 45 and 100 kPa corresponded to the predicted values from the OSR experiment. Further, more than 80% oxygen utilization efficiency (OUE) was achieved under all operational conditions, indicating effective oxygen uptake by nitrifying bacteria under oxygen-depleted conditions. Based on these experiments, the MABR was shown to be a controllable nitrification system, and to be able to provide a reaction space for nitrification in a membrane-attached biofilm without altering the bulk conditions.