A bench-scale submerged membrane activated sludge process using a microfiltration type hollow fiber membrane module and synthetic wastewater was operated at different BOD loadings and sludge retention times (SRT), and various filtration resistances were investigated. At short SRT, because of unstable biological growth and incomplete metabolism of organic matter, the suction pressure and the filtration resistances of the membrane and adhered sludge (cake) increased rapidly and randomly with treatment time, and thereafter destabilized the process. At long SRT and low BOD loading, low and steady suction pressure and filtration resistances were maintained for a long time. Almost all microorganisms in the bioreactor were in the stationary phase at SRT=500d (without sludge withdrawal), while the cake filtration resistance was very small and the microorganisms had better filtration characteristics.