In this investigation, we report the synthesis of biodiesel using benzimidazolium-based brønsted acid ionic liquid (BBAIL) catalyst under the influence of ultrasonication. The prepared BBAIL catalyst was characterized by Fourier transform infrared and NMR spectroscopy techniques, and its acidity was determined by the Hammett method with 4-nitroaniline as the indicator. Ultrasonicator horn (22 kHz, 500 W) was used in this work with an on-off cycle of 50-20 s at 70% amplitude. The highest biodiesel yield of 96% was achieved by ultrasonication when 1:10 molar ratio of castor oil to methyl alcohol was used at 50 °C temperature with 9 mol % of the catalyst in just 90 min, which is about 10 times lesser than the process without ultrasonication. At similar conditions, 96% biodiesel yield was obtained in 14 h without ultrasonication. In summary, ultrasonication proved to be an efficient way to improve biodiesel synthesis in less time and BBAIL showed excellent activity toward the conversion of glycerides to synthesize biodiesel. Other important highlights are easy separation of the catalyst and recyclability up to three cycles with small decrease in its activity. © 2018 American Chemical Society.