Flavonols from Ginkgo biloba and catechins from Camellia sinensis are poorly absorbed from the gut but extensively degraded by intestinal microflora into phenolic acids, whose antioxidant potential is poorly understood. We applied an improved Trolox equivalent antioxidant capacity (TEAC) method (ABTS radical cation decolorization assay) to establish the TEAC values of main metabolites from G. biloba flavonol-glycosides and green tea catechins. The results showed that the TEAC values of these metabolites varied: among G. biloba metabolites, 3,4-dihydroxy phenyl acetic acid had a relatively high TEAC value (2.16 ± 0.15 mM), while none of the green tea catechin metabolites had TEAC values comparable to their precursor (e.g., epigallocatechin gallate had a TEAC value of 4.81 ± 0.21 mM). The antioxidant capacity of these phenolic acids was mainly dependent on the presence of an o-hydroxy group, and influenced by the length of the alkyl chain, position of the hydroxy group, methoxylation, and substituents (such as carboxyl or glycine conjugation), which relate to electron-donating or withdrawing properties (consistent with Hammett's correlation). Considering the urinary levels of these metabolites after intake of G. biloba flavonol-glycosides or green tea catechins, these metabolites may contribute to the body's antioxidant potential.