A new cytotoxic triterpene hydroperoxide (3) was isolated from the methylene chloride extract of the aerial part of Youngia japonica together with four known triterpenes (1, 2, 4 and 5). Their structures were identified by means of physico-chemical and spectral data to be $eta$-amyrin acetate (1), taraxasterol (2), 2l$alpha$-hydroperoxy-taraxasterol (3), oleanolic acid (4) and ursolic acid (5). Compounds 3 and 5 showed moderate cytotoxicity against five tumor cell lines.