Transesterifications of six vegetable oil samples in supercritical methanol were studied without using any catalyst. Methyl esters of vegetable oils have several outstanding advantages among other new-renewable and clean engine fuel alternatives. The variables affecting the methyl ester yield during the transesterification reaction, such as molar ratio of alcohol to vegetable oil and reaction temperature, were investigated. Compared to no. 2 Diesel fuel, all of the vegetable oils are much more viscous, while the methyl esters of vegetable oils (biodiesels) are slightly more viscous.