Alkaline transesterification of Silybum marianum seed oil to biodiesel using methanol and ethanol was studied. The two methods used were conventional stirring (600 rpm) and ultrasonication (40 kHz). Oil was extracted from the seeds, followed by physico-chemical properties' determination and transesterification to biodiesel. The seeds contained 46% oil which had low free fatty acids (FFA) (0.68%). Linoleic acid (65.68%) was the main composition of the oil. Ultrasonication transesterification with methanol gave the highest yield (95.75%) after 20 min. Yields of methyl esters were higher than respective yields of ethyl esters. Using first order reaction kinetics model, the reaction rate constants were 2.3 X 10(-2) s(-1) and 7.0 X 10(-3) s(-1) for ultrasonication using methanol and ethanol, respectively. With the exception of oxidative stability (2.1 h) and iodine values (132-methyl and 133-ethyl esters), properties out of range but can easily be improved, the remaining properties including cetane number, flash point and the cold flow ones of both methyl and ethyl esters were similar and comparable to Chinese, ASTM and European Union standards. The findings of this study complement with the abundance of S. marianum oil at cultivation and silymarin industrial production as by-product indicates its potentially new non-edible feedstock for biodiesel. (C) 2014 Elsevier B.V. All rights reserved.