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Laccase, a type of polyphenoloxidase, was found to oxidize unsaturated triacylglycerol under mild reaction conditions in the presence of a simple natural mediator and mediator which is derived from a natural compound. The oxidized products were characterized by UV-and IR-spectroscopic analyses, ferric thiocyanate peroxide analysis and size exclusion chromatography. It was considered that conjugated diene peroxide units were formed in the triacylglycerol without substantial polymerization or degradation reactions of the main chain in the fatty acid residual group. As the mediator, 4-hydroxybenzoic acid, a simple natural phenol, which is secreted by white rot fungi, and the products derived from natural phenols were investigated. 4-Hydroxybenzaldehyde and methyl-4-hydroxybenzoate showed high oxidation activity comparable with synthesized mediators that have been previously reported. Trans-ferulic acid and 2,6-dimethoxyphenol that are not effective for the oxidation of anthracene showed a low oxidation activity for triacylglycerol in the present reaction system. The oxidation activity of the mediators used has some relationship to its oxidation potential. A bell-shaped curve with 4-hydroxybenzaldehyde as its apex may be drawn for the 4-substituted phenols when 4-hydroxybenzoic acid and alkyl-4-hydroxybenzoate were excluded.