Wastewater treatment characteristics in submerged membrane activated sludge process (SMASP) was studied for various sludge retention time (SRT) under a constant BOD loading (0.5 kg-BOD/(m3·d)).The SMASP operated at long SRT (500d) had low MLSS conversion ratio (0.11 g-MLSS/g-BOD).At short SRT (90, 120 and 160 d), TOC in the supernatant increased till 120~150 days and then the suction pressure raised owing to clogging of membrane pores.The complete assimilation of organic matter needed long SRT (500d) and the operation was stable.The effluent TOC (almost less than 5 mg/L) was less than supematant TOC in all runs irrespective of the SRT.When the SRT were increased, the corresponding values of OUR and ATP were decreased.The SMASP was stable at low microbial activity (OUR and ATP).In other words, at long SRT the system was stable.