Use of rapeseed soapstock to produce biodiesel is a sustainable choice, because it deals with waste stream use in biofuel synthesis and lowers the use of food grade oil as fuel feedstock. Rapeseed soapstock was acidulated, and fatty acid and glycerides containing acid oil was separated from water phase. Steam distillation – a mild purification method – was used to separate fatty acids from acid oil. Distilled fatty acids were subjected to sulfuric acid catalyzed esterification with methanol. The chosen reaction conditions were: 65 °C temperature, molar ratio of MeOH to FFA 20:1, 7.5 mol% H2SO4, and a reaction time 1 h. Esterification of distilled fatty acids proceeded with 98.3–98.5% conversion to FAME and product yield 93–95% from theoretical. The esterification reaction conditions were determined using lauric acid as model compound, the catalyst concentration was adjusted to be less than usually reported, so to avoid oxidation and side reactions. Use of ultrasound assisted synthesis was elaborated by comparing reaction of lauric acid at 25 °C temperature in ultrasonic bath and with stirring. Reaction was faster with stirring than use of ultrasound.