To study the chemical constituents of the medicinal plant Carpesium cernuum and their cytotoxic activity,ten compounds were obtained from the ethyl acetate fraction of 95% ethanol extract of C. cernuum by ap-plying various chromatographic techniques.Their structures were elucidated and identified as(5Z)-6-[(acetyloxy)methyl]-2-oxo-2,10,14-trimethylhexadeca-5,13-diene-11α-ol（1）,(2E,10E)-1,12-dihydroxy-18-acetoxy-3,7,15-trimethylhexadeca-2,10,14-triene（2）,(2E,6Z,10E,12R)-7-［(acetyloxy)methyl］-3,11,15-trimethylhexadeca-2,6,10,14-tetraene-1,12-diol（3）,(2E,6Z,11S,12R)-3,7,11,15-tetramethylhexadeca-2,6,14-triene-7-[(acetyloxy)methyl］-1,12,19-triol（4）,plaunotol（5）,S-(+)-dehydrovomifoliol（6）,pubinernoid A（7）,stigmasterol (8),aurantiamide acetate (9),E-p-hydroxycinnamic acid (10) respectively，based on the spectral data.Compound 1 is a new compound.The cytotoxic effect of compounds 1-4 on Human breast cancer MDA-MB-231 cell were investigated by MTT.Compound 2 exhibited strong cytotoxic activity with IC₅₀ values of 6.80 μmol/L,and compounds 1,3 and 4 had moderate cytotoxic activity.