Thirty-eight phenolic compounds (including 31 flavonoids) were examined for their DPPH radical-scavenging activities, and structure-activity relationships were evaluated. Specifically, the presence of an ortho-dihydroxyl structure in phenolics is largely responsible for their excellent anti-radical activity. 3-Hydroxyl was also essential to generate a high radical-scavenging activity. An increasing number of hydroxyls on flavones with a 3',4'-dihydroxyl basic structure, the presence of a third hydroxyl group at C-5', a phloroglucinol structure, glycosylation and methylation of the hydroxyls, and some other hydroxyls, for example 5-, and 7-hydroxyl in ring A, decreased the radical-scavenging activities of flavonoids and other phenolics.