Preparation of biodiesel from waste oils containing 72% of free fatty acids catalyzed by a novel Bronsted acidic ionic liquid 1-sulfobutyl-3-methylimidazolium hydrosulfate ([BHSO3MIM][HSO4]) was systematically investigated. The optimum molar ratio of methanol to waste oils, catalyst amount, reaction temperature and reaction time were 8/1, 10% (based on the mass of waste oils), 140 degrees C and 6 h, respectively, under which the obtained yield of biodiesel reached 94.9%. Also, [BHSO3MIM][HSO4] as a catalyst still retained around 97% of its original catalytic activity after successive re-use of 5 batches (6 h per batch), showing the excellent operational stability. Moreover, the acidic IL [BHSO3MIM][HSO4] was able to efficiently catalyze conversions of waste oils with different amounts of FFAs (free fatty acids) into biodiesel, and showed tremendous application potential. Therefore, an efficient and environmentally friendly catalyst is provided for the synthesis of biodiesel from waste oils with high acid value. (C) 2017 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.