Effects of Fe(III)/sulfate (Fe/S) ratio on organic carbon oxidation kinetics and microbial ecology of a novel Fe (III)-dosed anaerobic wastewater treatment system were investigated in this study. Fixed-film batch bioreactors under three Fe/S molar ratios (1, 2, and 3) yielded COD oxidation rates that increased with the Fe/S ratio, and estimated Michaelis-Menten model parameters V-max ranging in 0.47-1.09 mg/Lmin and K-m in 2503-3267 mg/L. Both iron and sulfate reducing bacteria contributed to the organics oxidation, and the produced sludge materials contained both biomass (32-45 wt.%) and inorganic precipitates from biogenic ferrous iron and sulfide (6855 wt.%). Spectroscopic and chemical elemental analyses indicated that the inorganic fraction of the sludge materials contained both FeS and FeS2, and had Fe/S stoichiometric ratios close to 1. Microbiological analyses of the biofilm samples revealed that the major putative iron- and sulfate reducers were Geobacter sp. and Desulfovibrio sp. along with noticeable N-fixing and fermentative bacteria. The COD oxidation rate had a positive correlation with the relative abundance of iron reducers, and both increased with the Fe/S ratio. A conceptual framework was proposed to illustrate the effects of Fe/S ratio on organics oxidation rate, microbial ecology and their interplays.