A membrane bioreactor (MBR) with gravity drain was tested for domestic wastewater for 65 days. Results showed that the effluent quality was excellent, and met with the reuse water standard of China (GB/T 18920-2002). Virus removal in the membrane separation process was investigated by employing coliphages T4 as a tracer. Two microfiltration membrane modules, with pore sizes of 0.22 and 0.1 m, were used to investigate their effects on virus rejection at the transmembrane pressure of 8.5 kPa. It was found that 0.1 m membrane had complete rejection of virus, and 0.22 m membrane had significant rejection of virus. In the longterm operation of this MBR, no significant difference was observed between both pore sizes because the virus concentrations of the effluent in both cases were in the same order. Effluent virus concentration at steady state of MBR running was less than 2 PFU/mL. The removal ratios of coliphage T4 in MF processes were more than 10 5.5 . The membrane surface deposits played an important role in the rejection of virus. The formation of cake clay on the membrane surface was the main cause of high rejection of coliphage T4 with MF of 0.22 m.