A lab-scale study was investigated to evaluate the effect of pure oxygen fine bubbles on membrane bioreactor (O-2-MBR) performance of treating coal gasification wastewater. Compared with conventional MBR using aeration source of air (CMBR), the removal efficiencies of COD and total phenols increased by 28% and 36%, and the organic compositions of treated effluent represented significant difference that was mainly attributed to the controlled the foam expansion and enhanced the enzymatic activities in O-2-MBR. Moreover, membrane fouling mitigation was observed in O-2-MBR, probably owing to the less EPS amount and larger PSD. It was notable that the pure oxygen with fine bubbles promoted marked evolution of bacterial community from CMBR to O-2-MBR, particularly, the bacterial community richness and diversity in O-2-MBR was lower than CMBR, and the genera Phycisphaera, Comamonas, Thauera and Ohtaekwangia composed the top four most relative abundance genera in O-2-MBR, giving the total relative abundance of 26.7%. (C) 2016 Elsevier Ltd. All rights reserved.