Sequencing batch reactor (SBR) was widely used in the treatment of various wastewater. The effects of low intensity ultrasound on the nitrogen removal performance of SBR were studied. The optimum operation conditions were determined to be 35 kHz, 0.15 W cm(-2), and irradiation time of 10 min. Compared with those of the control reactor, the organic, NH(4)(+)-N, NO(2)(-)-N and NO(3)(-)-N loads of the ultrasound enhanced reactor (UER) were improved by 16.5%, 35.0%, 41.7% and 61.9%, respectively. Increased 2,3,5-triphenyl tetrazolium chloride-dehydrogenase and nitrification activities were observed with sludge in UER. Furthermore, negligible negative effects of ultrasound irradiation on the settle ability and sludge concentration were found, which resulted in no decrease of the nitrogen removal performance of the UER. (C) 2011 Elsevier Ltd. All rights reserved.