A diarylheptanoid, (5S)-1,7-bis-(3,4-dihydroxyphenyl)-5- hydroxyheptane-3-one-5-O-β-D-xylopyranoside, named oregonin (1), was isolated from the of Alnus japonica (A. japonica), which is a species of the genus Betulaceae, growing throughout Korea, Japan and China. The structure was elucidated by various spectroscopic methods including negative and positive LC/MS, 1H-NMR, and 13C-NMR techniques or by comparison with authentic samples. In order to evaluate the anti-oxidative activities of oregonin (1) isolated from A. japonica, 1,1-diphenyl-2-picryhydrazyl (DPPH) radical scavenging activity and 2,2'-azino-bis[3-ethylbenzothiazoline-6-sulphonic acid] (ABTS) radical scavenging activity were measured in vitro. Oregonin from A. japonica exhibited potent DPPH and ABTS radical scavenging activities. A. japonica shows not only 1,1-diphenyl-2-picryhydrazyl (DPPH) radical scavenging activity and ABTS radical scavenging activity, but also apoptosis modulative effects. The present results indicate that A. japonica could be a hair-growth-promoting agent for cosmetic products.