In the course of screening for antioxidant compounds by measuring the radical scavenging effect on 1,1-diphenyl- 2-picrylhydrazyl (DPPH), a total extract of the twigs of Stewartia koreana (Theaceae) was found to show potent antioxidant activity. Subsequent activity-guided fractionation of the methanolic extract led to the isolation of six phenolic compounds, ampelopsin (1), catechin (2), proanthocyanidin-A2 (3), fraxin (4), (2R, 3R)-taxifolin-3-${eta}$-D-glucopyranoside (5), and (2S, 3S)-taxifolin-3-${eta}$-D-glucopyranoside (6), as active principles. Their structures were elucidated by spectroscopic studies. Compounds 1-6 were isolated for the first time from this plant. Among them, three compounds 1-3 showed the significant antioxidative effects on DPPH, and riboflavin originated superoxide quenching activity. In riboflavin-nitrobluetetrazolium (NBT)-light system, compound 1 showed better superoxide quenching activity than vitamin C.