Biodiesel has attracted increasing interest and has proved to be a good substitute for fossil-based fuels due to its environmental advantages and availability from renewable resources such as refined and waste vegetable oils. Several studies have shown that biodiesel is a better fuel than the fossil-derived diesel in terms of engine performance, emissions reduction, lubricity and environmental benefits. The increasing popularity of biodiesel has generated great demand for its commercial production methods, which in turn calls for the development of technically and economically sound process technologies. This paper explores the applicability of ultrasound in the optimization of low-cost feedstock – in this case waste cooking oil – in the transesterification conversion to biodiesel. It was found that the conversion efficiency of the waste oil using ultrasound was higher than with the mechanical stirring method. The optimized variables of 6:1 methanol/oil ratio at a reaction temperature of 30 °C and a reaction time of 30 min and 0.75% KOH (wt/wt) catalyst concentration was obtained for the transesterification of the waste oil via the use of ultrasound. Copyright: 2010 MDPI Publishing (Basel, Switzerland)