Effect of particle charge, size and fractal dimension under different pre-coagulation pHs and alum dosages on microfiltration (MF) membrane permeability and removal of humic acids (HAs) were studied. Both aggregates structure and size played important roles in membrane filtration. Under sweep coagulation condition, the insufficient low coagulant dosage of 10 mg/L alum resulted in low removal of HAs and the occurrence of serious membrane pore blocking by some small flocs with size corresponding to membrane pores. However with alum overdosing, the cake layer resistance increased by the direct fouling load from alum hydrolysis Al(OH)(3). Under complexation-charge neutralization coagulation, aggregates with low fractal dimension of loose structure improved permeate flux predominately when particles were near isoelectric point with alum dosages of 20 and 40 mg/L When alum increased to 60 mg/L and above, although the flocs fractal was much lower, the MF membrane permeability deteriorated severely because more small particles produce higher cake layer specific resistance as a result of a great charge repulsion force. Under the optimum pre-coagulation alum dosages of 20 and 40 mg/L, the membrane fouling was much more alleviative at pH 5 than pH 7 because the more loose aggregates formed under charge neutralization coagulation than sweep coagulation. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.