Effect of dissolved oxygen (DO) concentration on simultaneous removal of carbon and nitrogen by a membrane aeration/filtration combined bioreactor (CMBR) was investigated through experimental test, which used synthetic municipal wastewater. The results showed that the CMBR achieved a good simultaneous removal of carbon and nitrogen when DO concentration was around 0.5 mg/L Under this condition, the COD, ammonia and TN removal efficiency could reach to 94.5%,96.0% and 78.4%, respectively. When the DO concentration decreased to 0.1 mg/l, the limitation of oxygen led to the hampering of nitrification and low removal efficiency of ammonia and TN which were just 69.3% and 65.7%, respectively. When DO was set to around 2.0 mg/L or 4.0 mg/L, the COD and ammonia removal rate could be as high as 96%, but the TN removal efficiency was dropped to 50.4% and 26.4%, respectively. All of the above results suggested that controlling DO around 0.5 mg/L in CMBR was good for the removal of COD, ammonia and TN simultaneously. (C) 2009 Elsevier B.V. All rights reserved.