Antioxidative compounds contained in the leaves of Melissa officinalis L. (lemon balm) were investigated. Six major compounds, protocatechuic acid (or 3,4-dihydroxybenzoic acid, 1), 2-(3′,4′-dihydroxyphenyl)-1,3-benzodioxole-5-aldehyde (2), caffeic acid (3), rosmarinic acid (4), caffeic acid methyl ester (5), and rosmarinic acid methyl ester (6) were isolated from the extract of the plant and the 1,1-diphenyl-2-picrylhydrazyl radical scavenging activity was measured. Among them, compound 2 showed the most potent activity, about ten-fold that of ascorbic acid and of α-tocopherol. The compound had a characteristic 1,3-benzodioxole structure and was easily degraded into two molecules of protocatechualdehyde.