Nitrification performance of a nitrifying biofilm attached on silicone hollow fiber membrane was evaluated. The nitrogen surface loading was increased from 2.32 to 9.28 g/m(2)d in steps with the membrane-attached biofilm that was operated at an inlet air pressure of 98 kPa and hydraulic retention time of wastewater of 12 h. The nitrification rate of about 8.0 g/m(2)d was simultaneously obtained with the nitrification efficiency of about 90%. The results clearly demonstrated that the oxygen supply from the bottom of biofilm through an oxygen permeable silicone membrane largely contributed the nitrification performance and that the nitrification performance of the membrane-attached nitrifying biofilm was far superior to the conventional fixed biofilm processes.