In order to investigate the optimal C/N ratio for simultaneous nitrogen and carbon removal by a gas-water alternate membrane bioreactor(AMBR)treating wastewater,an AMBR in which two hydrophilic polypropylene hollow-fiber membrane modules was used for aeration and filtration alternately was constructed,and the effect of C/N ratio on simultaneous removal of nitrogen and carbon by AMBR treating synthetic wastewater was studied during the 150 days’ continuous operation.The results showed that under the condition that DO in the mixed liquor was 0.5 mg/L around,the influent COD and NH4+-N was(100-350)mg/L and 34 mg/L,and C/N ratio was in the range of 3-10,increase of C/N ratio had no influence on COD and NH4+-N removal.During the 150 days’ operation,COD and NH4+-N in effluent maintained 20 mg/L and 2 mg/L around,respectively.However,increase of C/N ratio was beneficial to TN removal by AMBR.When C/N ratio was more than 5,TN removal efficiency was more than 66%,and the effluent TN was lower than 15 mg/L,which met the First A-level requirement of national GB 18918—2002.