A goal of this study is to investigate organics, NH4+-N and PO₄^3--P removal at various sludge retention time(SRT) using A/O type MBR. Lab-scale submerged MBRs at SRT of 10, 15, 20 and 30 days were performed at constant permeate flux of 10.2 l/m2·h with hollow fiber microfiltration membranes. NH4+-N removal efficiency increased with SRT since a higher biomass concentration, which was derived by longer SRT. But PO4^3--P was hardly removed. Regardless of SRT change, COD removal efficiency was considerably high(average 85% about) througnout the experiment.