The influence of aeration intensity on membrane fouling in membrane bioreactors (MBR) was to investigate. Two lab-scale MBR with aeration intensity of 500 and 100 L/h were operated for 60 days at a constant permeate flux. The dead-end filtration tests were conducted to confirm the mixed sludge filterability under each condition. The effect of aeration intensity on relative molecular mass distributions of SMP, the particle size distributions (PSD) of flocs and the quantity of extracellular polymeric substances (EPS) in MBRs were also tested. The results show that large aeration intensity has a negative influence on the filterability of mixed sludge in MBR. Further research finds that the large aeration intensity results in the increase of soluble microbial products (SMP) with a relative molecular mass (M(r)) > 10 000 in supernatant, which obviously affects the filtration of mixed sludge. It is also found that small particles in the range of 1-10 microm and the concentration of EPS in flocs remarkably increase under high aeration intensity of 500 L/h.