Catechins are a group of polyphenolic compounds abundantly contained in green tea. It is well known that catechins have multiple biological activities including anticarcinogenic and antiinflammatory effects. These protective effects are due to their antioxidative activities by scavenging free radicals. All C-H and O-H bond dissociation enthalpies (BDE's) in catechins ((-) -epicatechin, (-) -epigallocatechin, (-) -epicatechin gallate, (-) -epigallocatechin gallate) were calculated by semiempirical molecular orbital method using SPARTAN program. The BDE's of benzyl hydrogens (C-2 position in catechins) are found to be quite low. This result suggests that abstraction of benzyl hydrogen is a crucial step for antioxidant activity. This is also supported by the reported results of LC/MS/MS and spectrophotometric analysis of reaction intermediate from catechins treated with AAPH.