The coal-based solid acid catalysts are synthesized by incomplete carbonization followed by sulfonation approach and their activities in catalyzing esterification of oleic acid with methanol are subsequently investigated. For characterizations of catalyst, N-2 adsorption and desorption, ultimate analysis, XRD, SEM, XPS, TG, FTIR and acid amount test are employed. The synthesis conditions are optimized as carbonization at 400 degrees C for 1 h and sulfonation at 105 degrees C for 2 h, where acid amount of 1.57 mmol . g(-1) is measured. Ultrasound irradiation is introduced into the esterification reactor, and the effects of different operation parameters, including the ultrasonic power, catalyst dosage, molar ratio of methanol to oleic acid, reaction temperature on the esterification efficiency under ultrasound-assisted as well as sonication-free method, have been investigated. The results demonstrate that after passing the given reaction duration of 1 h, 91.4% of esterification efficiency is obtained under the optimized conditions (i.e., ultrasonic power of 270 W, catalyst dosage of 6 wt.%, methanol to oleic acid molar ratio of 10, and reaction temperature of 67 degrees C). 87.9% of conversion is achieved under the same operating conditions without ultrasonic irradiation. Moreover, esterification experiments indicate that this synthesized coal-based solid acid catalyst possesses favorable catalytic activity and satisfactory recyclability in biodiesel production. (C) 2017 Elsevier Ltd. All rights reserved.