A submerged membrane bioreactor (MBR) treating synthetic wastewater was continuously operated to investigate the impacts of floc size on membrane fouling. Particle size distribution (PSD) analysis showed that almost all the sludge flocs in sludge suspension had size larger than 1 gm, which corresponded to low pore clogging fouling. Thermodynamic analyses showed that adhesion of sludge flocs on membrane surface needed to overcome a repulsive energy barrier. Decrease in floc size slightly increased the specific energy barrier, but highly increased the attractive specific interaction energy in contact, and as whole, facilitated adhesion of small flocs to the membrane surface. Presence of biopolymer matters in sludge suspension remarkably gave rise to the cake resistance. This result could be explained by the osmotic pressure mechanism. Decrease in floc size would greatly increase bath of hydraulic cake resistance and osmotic pressure-induced resistance. The findings shed significant light on membrane fouling control. (C) 2015 Elsevier B.V. All rights reserved.