To investigate the wastewater treatment efficiency of an anoxic tank-aerobic MBR system,COD,ammonia and total nitrogen (TN) removal rate were measured at different organic loadings (OL) of raw sewage (in COD,315. 8,407. 5 and 507. 7 g·m-3·d-1). PCR-DGGE and 16S rDNA were used to detect microbial community structures in the system. Results show that increasing OL can enhance the removal rate of COD from 65. 9% to 85. 95% and that of TN from 18. 5% to 69. 5% ,but it has little impact on ammonia removal. The system has advantages on both low-organic-carbon-dependant TN removal and excess sludge control. Changes of OL obviously influence system’s microbial community structures. The system is robust and highly efficient under moderate OL conditions,with the highest Shannon diversity index (0. 95) of all the OL conditions.