Abstract: The effect of powdered activated carbon (PAC) on the mixed liquor characteristics and membrane filtration resistance was studied by using two membrane bioreactors（Reactor 1 no PAC and Reactor 2 added PAC 500 mg/L）under the same influent and operating conditions. The results showed that adding PAC could effectively slow down the increase of viscosity，extracellular polymeric substances（EPS），tiny particulate content of the mixed liquid，and membrane filtration resistance. The membrane pore jamming model could be merged into the membrane self-resistance model，since the membrane pore jamming only worked in a very short time (≤1 min) and the particles bigger than the membrane pore and smaller than 1μm were the major matter of gelatin layer，which caused main filtration resistance.