In this paper, an NiSO4/SiO2 catalyst was prepared by impregnating an SiO2 support with an aqueous solution of NiSO4 center dot 6H(2)O. Through an esterification reaction using various catalysts including zeolite, SiO2, and H2SO4, free fatty acids (FFAs) in brown grease were converted to fatty acid methyl esters (FAMEs) as biodiesel products that were analyzed by gas chromatography-flame ionization detection, gas chromatography-mass spectrometry, and nuclear magnetic resonance. To optimize the esterification conditions, a molar ratio of methanol/brown grease from 1.57 to 3.70, a weight ratio of catalyst/brown grease from 0.75 to 1.75%, and reaction times from 30 min to 3 h at room temperature and atmospheric pressure were investigated. The chemical compositions of FFAs in brown grease were characterized as n-hexadecanoic acid (C16H32O2), 9,12-octadecadienoic acid (C18H32O2), and octadecanoic acid (C18H36O2). Biodiesels obtained from these FFAs included hexadecanoic methyl ester (C17H34O2), 9,12-octadecadienoic methyl ester (C19H34O2), 9-octadecenoic methyl ester (C19H36O2), and octadecanoic methyl ester (C19H38O2). Among the catalysts that were tested, the order of the catalytic activity was H2SO4 > NiSO4/SiO2 > zeolite > SiO2. Through the experiments conducted in this research, the optimized esterification conditions proved to be a methanol/brown grease molar ratio of 2.64, a catalyst/brown grease weight ratio of 1.00%, and a reaction time of 30 min. The use of NiSO4/SiO2 at room temperature might be significant for biodiesel production because it may allow the catalyst to be reused after recycling and lower the cost of biodiesel production without providing additional heating for the conversion of FFAs. DOI: 10.1061/(ASCE)EY.1943-7897.0000086. (C) 2013 American Society of Civil Engineers.