The microbial electrolysis cell (MEC) is a promising technology for anaerobic wastewater treatment and energy recovery. Applied voltage affects the activities of microorganisms in MEC and further impacts methane generation. Therefore, we explored the effects of applied voltages on the performance of MEC and the microbial activities under anaerobic conditions. With COD concentration of 2-3 g/L in influent, COD removal efficiency and methane yield increased when the applied voltage was lower than 0.8 V and decreased when the applied voltage was 1.0 and 2.0 V. Lactic dehydrogenase (LDH), as an indicator for cell rupture, its activity increased 14% and 10% under 1.0 and 2.0 V in anode and increased 35% and 41% in cathode, adenosine triphosphate (ATP), as an indicator for metabolism, its activity decreased sharply, 27% and 55% in anode while 46% and 66% in cathode, under the two voltages respectively, comparing to the control. Results indicated that the higher voltage led to the greater plasmatorrhexis, lower growth rate and lower metabolic activity. The destroyed cell membrance resulted in slower growth and metabolism rate, which meant the decrease of COD removal efficiency and methane yield, when the voltage higher than 0.8 V conditions. In conclusion, the optimal applied voltage for wastewater treatment was 0.8 V. Information provided will be useful to design reactor and maintain industry practice. (C) 2015 Elsevier B.V. All rights reserved.