An equipment of a submerged membrane activated sludge process without sludge withdrawal was operated at three BOD loadings (0.25, 0.5 and 0.6 kg-BOD/(m(3.)d)) to find out its stable operational condition. The stabilized and final values of MLSS were increased with BOD loadings and the conversion ratios (0.04-0.15 kg-MLSS/kg-BOD) of substrate to biomass were much smaller than the conventional activated sludge process. The supernatant TOC in the bioreactor was low in all runs and then organic matters were not accumulated. At low BOD loadings (0.25, 0.5 kg-BOD/(m(3.)d)), suction pressure was constant or increased very slowly, but at the highest BOD loading (0.6 kg-BOD/(m(3.)d)) the pressure and total filtration resistance were increased abruptly and failed the operation rather soon. The good stable operational condition without sludge withdrawal was obtained at BOD loading below 0.5 kg-BOD/(m(3.)d).