<p>Tung oil contains approximately 78% <i>α</i>-eleostearic acid (9<i>c</i>, 11<i>t</i>, 13<i>t</i>-octadecatrienoic acid), 9% linoleic acid, and 7% oleic acid. This oil is easily oxidized because of the three conjugated double bonds in <i>α</i>-eleostearic acid; however, it is stable within the tung seed. Previously, we isolated and identified a catechol-type lignan and neolignans from tung seeds, which exhibited potent antioxidant activity. Catechol compounds were specifically present in the seed coat of tung trees. In this study, we quantified the catechol-type lignans in the seed coat of Euphorbiaceae oil-producing plants and found that the plant seeds containing easily oxidized fatty acids contained higher amounts of catechol-type lignans in their seed coat. The antioxidant activity assay of phenolic compounds indicated that catechol-type compounds have higher antioxidant activity than monophenol-type compounds, and that the two hydroxy groups in the <i>ortho</i>-position of the benzene ring are important for enhancing antioxidant activity. Analysis of the antioxidant mechanism by ¹³C nuclear magnetic resonance (¹³C NMR) spectroscopy showed that isoamericanol A captured the 2, 2-diphenyl-1-picrylhydrazyl (DPPH) radical and preferentially changed it to the quinone structure. The oil oxidation inhibition test revealed that <i>α</i>-tocopherol inefficiently inhibited the oxidation of tung oil, and that catechol compounds with a higher antioxidative activity than <i>α</i>-tocopherol would be required to prevent the oxidation of tung oil. These results show that tung seeds synthesize catechol-type lignans in the seed coat to prevent the oxidation of tung oil in the endosperm.</p>