The behavior of various iron species (e. g. ferrous, prehydrolyzed(ph)-ferrous ion and ferric ion) as a coagulant and effects of inert mineral on the coagulation process combined with direct microfiltration (MF) membrane process was studied for humic acid (HA) removal. When 0.01 mM to 1 mM of the various iron salts added to remove HA at pH 3 to pH 9, HA removal efficiencies were evaluated. The maximum complexation capacity of ferric and ferrous salt for HA removal was 739.2 mg DOC/L-mM and 472.3 mg DOC/L-mM, respectively. The optimum pH region for HA removal ranges from pH 4.5 to 6.3 as ferric salts are added, pH 3-4 and pH 6-7 are optimum regions for ferrous and ph-ferrous salts, respectively. The optimum pH region of ph-ferrous was broader than that of ferrous salts. The influence of inert mineral particles (e.g. glass, bentonite, and zeolite) on iron salts coagulation with HA was also examined. Results showed that the pH range for HA removal dramatically expanded when the inert particles were added with iron salts even though the inert mineral only adsorbed HA in acidic condition below pH 5. Moreover, added inert particles increased the critical flux by 700 L/m(2)-h (TMP 101 kPa), which is seven times higher than that with ferric salt added alone.