Phytochemical works on the aerial parts of Aster oharai (Compositae) led to the isolation of a new sesquiterpene hydroperoxide, 7$alpha$-hydroperoxy-3, 11-eudesmadiene (2) and seven known compounds, teucdiol B (1), $alpha$-spinasterol (3), oleanolic acid (4), $alpha$-spinasterol 3-Ο-$eta$-D-glucopyranoside (5), methyl 3,5-di-Ο-caffeoyl quinate (6), 3,5-di-Ο-caffeoylquinic acid (7), 3,4-di-Ο-caffeoylquinic acid (8). The chemical structures of 1-8 were established by chemical and spectroscopic methods. Compound 2 showed cytotoxicity against cultured human tumor cell lines in vitro, SK-OV-3 (ovarian), SK-MEL-2 (skin melanoma), and HCT15 (colon) with $ED_{50}$ values ranging from 3.86-17.21 $mu$g/mL.