Alnus japonica Steud (Betulaceae) has long been used as a Korean traditional medicine for gastric disorders, hepatitis, and fatty liver. As a part of our study on the identification of secondary metabolites of naturally occurring bioactive compounds, we isolated 1,7-bis(4-hydroxyphenyl)-3,5-heptanediol (1), 5-hydroxy-1,7-bis(4-hydroxyphenyl)-3-heptanone (2), 5,3-dihydroxy-7,4-dimethoxyflavone (3) and 3,5,7,3,4-pentahydroxyflavone (4) from the dichloromethane and ethylacetate-soluble fractions of Alnus japonica Steud. These compounds showed significant antioxidant activity in a concentration-dependent manner. The IC?? values of compounds 1, 2, 3 and 4 were 30.1, 37.4, 20.2 and 13.7 §¶/mL, respectively, through the scavenging capacity of 1,1-diphenyl-2-picrylhydrazyl radical assay.