From the ethyl acetate fraction of the roots of Ostericum koreanum, a new chromone, 11-hydroxy-sec-O-glucosylhamaudol (1) along with the known compounds: four chromones, three coumarins, six phenolic compounds, and three quinic acids were isolated. These compounds were assessed for antioxidant activities in the DPPH radical and superoxide anion radical scavenging assay systems. Among isolates, 4-(2-hydroxy-vinyl)-benzene-1,2-diol (12) showed the most potent DPPH radical scavenging activity (IC(50)=4.80+/-0.62 mug/ml) and superoxide anion radical scavenging activity (IC(50)=11.05+/-0.83 microg/ml) in the xanthine/xanthine oxidase system. The antioxidant activities of 12 were comparable to those of quercetin and luteolin.