Three new sesquiterpene hydroperoxides, 1-[3-(2-hydroperoxy-3-methylbut-3-en)-4-hydroxyphenyl]ethanone (2), 7beta-hydroperoxy-eudesma-11-en-4-ol (3), and 7alpha-hydroperoxymanool (4), together with three known compounds, germacrone (1), ent-germacra-4(15),5,10(14)-trien-1alpha-ol (5) and teucdiol A (6) were isolated from the aerial parts of Aster spathulifolius (Compositae). Their structures were characterized using chemical and spectroscopic methods. The isolated compounds were tested for their cytotoxicity against five human tumor cell lines in vitro using a SRB method. The two new compounds, 3 and 4, showed moderate cytotoxicity against human cancer cells with ED50 values ranging from 0.24 to 13.27 microg/mL.