In this study, organic stabilization and nitrogen removal were investigated using a household-type hollow fiber membrane separation bioreactor of 1.9L volume. The process employed direct solid-liquid separation by the hollow fiber membrane module and an aeration tank. There was a distinct effect of solid content on the membrane flux. The steady water flux decreased as MLSS concentration increased from 3,500 to 13,300㎎/ℓ. The reduction of influent COD aver 95% was obtained in the system without sludge wastage. However, by-products of cell degradation could accumulate and have adverse effect in the biological reactor if sludge was not wasted. SS was completely removed from the effluent so that clean effluent could be obtained by the membrane bioreactor. Enteric bacteric such as E. coli was not detected in the effluent. By the ultrafiltration membrane bioreactor, TKN was removed up to 90% or more by simultaneous nitrification and aerobic denitrification. Endogenous denitrification rate of 0.0139g NO₃-N/g VSS.day was observed.