Methanol extracts from 20 Korean plants were screened for their antioxidant and radical scavenging activities. The methanol extract of Geum japonicum (Rosaceae) was found to be the most active one. Purification of this extract by liquid/liquid partitioning followed by open column chromatography (Sephadex LH-20), and finally RP-C18 HPLC led to the isolation and identification of 3,4,5-trihydroxybenzaldehyde (THBA) as the active principle. Compared to four other standard antioxidants which were butylatedhydroxytoluene (BHT), butylatedhydroxyanisole (BHA), α-tocopherol, and rosmarinic acid, THBA was found to be significantly more active in the radical scavenging assay using 2,2-diphenyl-1-picrylhydrazyl (DPPH). Similar results were obtained in the Rancimat test with both lard and palm oil as the substrate. The THBA content in dried samples of Geum japonicum was found to contain 140.7 mg/kg in the leaves, 240.5 mg/kg in the stems, and not found in the roots.