7-Isopentenyloxycoumarin (1) was isolated from Heracleum lanatum MICHX. (Umbelliferae). Compound 1 inhibited phospholipid metabolism and Epstein-Barr virus activation caused by a potent tumor promoter. In an in vivo experiment, topical application of 1 suppressed skin-tumor-formation induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) in 7,12-dimethylbenz[a]anthracene (DMBA) initiated mice. And it also suppressed ornithine decarboxylase activity stimulated by TPA on mouse skin. These results indicated that 7-isopentenyloxycoumarin is one of the effective compounds from natural resources for treating skin tumor formation.