The characteristic of aggregates pre-coagulated by inorganic monomer alum, polymer aluminium chlorohydrate(ACH) and polyaluminium, chloride(PACI) coagulants impose major impact on the removal of humic acids (HAs) and the reduction of microfiltration (MF) membrane fouling. The fractal dimension of flocs formed by ACH and PACI is higher than that by monomer alum, indicating Keggin structure produced by polymer coagulants is much more compact compared with hexameric ring structure of alum hydrolysis species. Correspondingly, cake layer specific resistance is far higher and the MF membrane flux deteriorates much more severely when pre-coagulated by ACH and PACI than by alum. Moreover, the higher basicity contains in PACls, the cake layer fouling is more serious for producing more proportion of dense hydrolysis species Al-13. Thus, the polymer coagulant ACH and PACI seems not adapt to the pre-coagulation-MF process for cake layer resistance increase two to three times although they save 60-70% dose in comparison with alum for HAs removal. Additionally, for three Al-based coagulants under sweep coagulation condition, insufficient dose result in lower HAs removal and produce more small particles caused higher cake layer specific resistance according to Carman-Kozeny relationship. On the other hand, coagulant hydrolysis species as direct contaminant loading aggravated cake resistance on the MF membrane when overdosed. The optimum dose should keep the minimum to provide better HAs removal efficiency, and produce lower cake layer specific resistance and higher membrane filterability for pre-coagulation-MF hybrid process. Crown Copyright (c) 2008 Published by Elsevier B.V. All rights reserved.