A recently developed Membrane Fouling Reducer (MFR) is an effective coagulating and flocculating materials made of water-soluble cationic polymers. Continuous addition of MFR to MBR enables mitigation of membrane fouling and sludge production. However, the mechanism how MFR reduce membrane fouling has not been reported. The purpose of this paper is to investigate the role of MFR fouling in the MBR system. In particular, the effect of bioflim structure deposited on membrane surface such as porosity, biovolume, average run length, PSD (Particle Size Distribution), and soluble and bound EPS (Extracellular Polymer Substances) on the membrane fouling was elucidated. The addition of MFR enhanced the membrane permeability only when the dose of MFR was below the critical value. The biofilm porosity of the Control reactor was lower than that of the MFR reactor whereas the biovolume in MFR reactors were higher than that in Control reactor. Therefore, the biofilm in the MFR reactor became less dense and the pore volume became greater, leading to higher permeability. Consequently MFR could alter the biofilm structure, and thus the membrane filtration performances were affected by MFR.