Acer tegmentosum is a type of deciduous tree that grows in Korea. It has been used in traditional medicine for the treatment of hepatic disorders. In this study, chromatography fractionation and isolation have been successfully used to yield 15 compounds, including 10 flavonoids, 4 phenylethyl glycosides, and 1 other glycoside. Their structures were determined on the basis of their physical and spectral properties [(1)H and (13)C nuclear magnetic resonance (NMR) and mass spectrometry (MS)] and by comparison of these results to similar data in the literature. The total peroxyl radical-scavenging capacity of each isolated compound was evaluated. Among them, the most active components belong to the flavonoids. Among these, quercitrin (1), 6-hydroxy-quercetin-3-O-galactose (6), and (+)-catechin (8) showed stronger activity than the positive control Trolox.