In order to investigate hydrothermal oxidation of unsaturated carboxylic acids, oleic acid and linoleic acid were oxidized at 573K with an insufficient oxygen supply. On the basis of intermediate oxidation products identified by GC/MS and HPLC analyses and the free-radical reaction mechanism, the hydrothermal oxidation pathways of unsaturated carboxylic acids are discussed. The rupture at carbon-carbon double bond by the addition of a peroxide radical to the double bond may be a major mechanism. The mechanism may also lead to the rupture of carbon-carbon bond at the single bond near to the double bond. Subsequently, the oxidation of the formed saturated mono- and di-carboxylic acids and aldehydes proceeded with the consecutive oxidation of higher to lower molecular weight carboxylic acids. If these substances converted to low molecular weight esters in a manufacturing process of biodiesel fuel(BDF), produced BDF was expected that the depression of cloud point and the improvement of other cold flow properties.