To investigate the effects of ozonation on minimizing the excess sludge and enhancing the nitrogen removal in an effluent, batch and continuous experiments in two MBRs with and without sludge ozonation ( namely combined and reference run) were carried out. Through ozonation at a dose of 0.16 mg O3/mg MLVSS,53.1% of the treated MLVSS was solubilized, and soluble SCOD/TN ratio of ozonized sludge (OS) was about 8.6 due to the release of cellular nitrogen-contained materials and SCOD loss by ozone mineralization. In addition, the results of batch nitrification and denitrification tests with OS supernatant indicated that solubilized sludge could act as a reducing power for denitrification and a nitrogen source for nitrification. 40-day operation of two MBR systems demonstrated that the recirculation of OS into a bioreactor enabled the combined system have two advantages over the control one. The observed sludge yield (Yobs) was decreased from 0. 13 to 0. 06g MLSS/g COD, while the nitrogen removal was increased from 64.6% to 72. 3%. And sludge ozonation elevated the inorganic fraction of MLSS, but did not impact sludge activities.