Flavonoids are a large family of polyphenolic compounds synthesized by plants that have a common chemical structure. Flavonoids may be further divided into subclasses, Anthocyanidins, Flavan-3-ols, Flavanones, Flavonols, Flavones, Isoflavones. Flavonoids have antioxidant and antiradical activities. The antiradical efficacy of flavonoids have established structure-activity relationships (SAR) of the antioxidant activity. Reactivity of flavonoids is highly dependent on the configuration of OH groups on the flavonoid B and C rings, there being little contribution from A ring to antioxidant effectiveness. This short article presents the current knowledge on structural aspects on their free radical reactions of flavonoids.