Ultrasound is considered as one of the most attractive energies to assist biodiesel transesterification. The objective is to reduce the reaction time while saving energy. In the present study, four oils showing a wide range of fatty acid composition have been transesterified using ultrasound as auxiliary energy. Samples included unsaturated fatty acids (rapeseed and soybean) oils and saturated fatty acids (coconut and palm) oils. Transesterification reactions were conducted in batch and an ultrasound probe working under a fixed frequency of 20 kHz was used to facilitate the overall process. According to the design of experiments, variable duty cycle and amplitude, besides different concentrations of basic catalyst (KOH) and methanol-to-oil molar ratio were studied. The optimal fatty acid methyl ester (FAME) values were 95.03% for rapeseed biodiesel, 94.66% for soybean biodiesel, 81.37% for coconut biodiesel and 93.08% for palm biodiesel. A response surface methodology (RSM) was applied to determine the reaction parameters with a significant impact on response variables. Results showed that the length of chain and in a lower extent unsaturation degree of fatty acids have a significant effect on ultrasound assisted transesterification. In this sense, oils with higher unsaturation degree and long hydrocarbon chains provide higher FAME yields and lower glyceride concentrations than those with saturated fatty acids and short hydrocarbon chains. (C) 2014 Elsevier Ltd. All rights reserved.