Abstract Many attempts have been made to control fouling in membrane bioreactors (MBRs) to sustain permeability through relaxation, backwashing and chemical cleaning. Sodium hypochlorite is one of the most useful chemical agents used to control fouling in chemically enhanced backwashing (CEB) of MBRs. However, sodium hypochlorite has the potential to adversely affect microorganisms because CEB is usually carried out within the bioreactor. Hence, this study investigated how sodium hypochlorite influences activated sludge from the viewpoint of microorganism properties and suggests the limit of sodium hypochlorite dosage. Activated sludge was evaluated using the substance removal rate, extracellular polymeric substances (EPSs), particle size distribution (PSD) and permeability. It was shown that 1mg-NaOCl/g-MLVSS had a slight superiority in nutrient removal compared to 5mg-NaOCl/g-MLVSS. On the other hand, the NH4-N removal ratio of 1mg-NaOCl/g-MLVSS was bigger than that of 5mg-NaOCl/g-MLVSS in nitrification for 3h from the time of sodium hypochlorite addition. While 1mg-NaOCl/g-MLVSS showed a similar tendency with the reference sludge in the case of EPSpolysaccharide, 5mg-NaOCl/g-MLVSS generated EPSpolysaccharide 1.5 times as much as that of 1mg-NaOCl/g-MLVSS. Also, according to particle size distribution, the average floc size of 5mg-NaOCl/g-MLVSS was changed from 43.63 to 36.98μm. As a result, the sludge of 5mg-NaOCl/g-MLVSS had a much higher potential for increase than the reference sludge in membrane fouling. Moreover, a microfiltration experiment confirmed that 5mg-NaOCl/g-MLVSS had a double rate increase of resistance compared with 1mg-NaOCl/g-MLVSS.