Treatability and pressure difference in ultrafiltration was investigated in sequential anaerobic/aerobic activated sludge wastewater treatment system. Ultrafiltration had advantages for sludge separation than secondary sludge settling tank in biological wastewater treatment system. Clean and steady water quality in effluent was achieved but the flux was decreased with the lapse of time. Accordingly, the influence of control parameters of backwashing intervals and various loading rate to treatment efficiency and pressure difference to sustain steady flux were studied for the combination process of activated sludge and ultrafiltration membrane. The effluent BOD concentration showed less than 2㎎/l and T-N resulted in 11.5∼21.2㎎/l in this system. When organic loading rate was 0.5㎏BOD/㎥·d with less than 0.04㎏/c㎡ pressure difference the system could be controlled to sustain 0.2㎥/㎡·d flux. Specifically, hydrophilic membrane showed less than 0.02㎏/c㎡ pressure difference. Flux of 0.2㎥/㎡·d required 0.04㎏/c㎡ pressure difference but higher pressure difference was required to raise flux. Outside pressure infiltration and suction infiltration were compared. 0.25∼0.35㎏/c㎡ pressure difference was required to earn 0.2㎥/㎡·d of flux for outside pressure infiltration but only 0.04㎏/c㎡ pressure difference was required for the same flux of in the case of suction infiltration. In conclusion, there were no difference in effluent water quality between filtration by hydrophobic membrane and hydrophilic membrane but lower pressure difference was required by hydrophilic membrane to earn steady flux rate. When flux and organic loading rate in the reactor was raised, the required pressure difference was dramatically increased. In the case of longer backwashing intervals pressure difference tends to build up.