A prenylated coumarin, osthenol (1) and a sesquiterpene, bisabolangelone (2) have been isolated as active principles with 5α-reductase type 1 inhibitory effects in LNCaP cells from the roots of Angelica koreana Max. by bioassay-guided chromatographic fractionation. Osthenol exhibited a highly potent inhibitory activity on 5α-reductase type I in LNCaP cells with an IC50 value of 0.1 μg/ml, which is about 200 times more potent than the positive control, finasteride (IC50 = 19.8 μg/ml). Bisabolangelone also inhibited the activity of 5α-reductase type I in LNCaP cells (IC50 = 11.6 μg/ml), indicating that these compounds are possible candidates for the development of new drugs to treat human endocrine disorders associated with overproduction of DHT by 5α-reductase type I. In addition, four compounds isooxypeucedanin, oxypeucedanin hydrate, oxypeucedanin and isoimperatorin were also isolated and found to be inactive in the 5α-reductase assay systems used in the present study.