Membrane Bie-Reactor (MBR) system offers several advantages over conventional activated sludge process. But the major hurdle in the extensive use of this process is the continuous reduction of permeation flux caused by membrane fouling. The aim of this study is to investigate membrane fouling characteristics in the MBR process. As MLSS decreased, Rc decreased. The cake resistance (R_(c)) was predominant even though MLSS fell down to 90㎎/L. Composition of ECP was investigated using the heating method. Potpaccharide was predominant (58%) Over the other constituents (DNA, RNA. Protein). The selective oxidant for sugars, sodium periodate was dosed to activated sludge to partialty destruct the floc matrix. and then ultrafiltraon of the suspension was canied out with a control (without periodate treatment). R_(c) and R for the sludge treated by periodate increased and Roc size decreased. This floc breakage was due to an oxidation of ECP by periodate. Destruction of floc structure caused by partial oxidation of ECP played a key role in increase of R_(c) and R_(f) respectively. The breakage of ECP was confirmed by GFC (Gel Permeation Chromatograph).