The seeds of Vernonia anthelmintica afforded three new elemanolide dimers, vemodalidimers C, D, and E, and four known elemanolides. Their structures were elucidated based on extensive spectroscopic analysis (IR, HR-ESIMS, 1D- and 2D-NMR), and the absolute configurations of the new dimers were determined by comparison of the experimental and calculated electronic circular dichroism spectra. Cytotoxicity of the isolated compounds against four human tumor cell lines was assayed. Vernodalidimers C, D, and E exhibited strong cytotoxicity against T47D cell line with IC50 values of 5.58, 0.95, and 12.75 mu M, respectively.