Three new pseudo-disesquiterpenoids, vernodalidimer F - H (1 - 3), which are formed by esterification of two sesquiterpenoids, along with a new sesquiterpenoid (4) and two known sesquiterpeonids (5, 6) were isolated from the seeds of Vernonia anthelmintica. Their structures were elucidated by NMR data. The absolute configurations of 1-3 and 5 were determined by comparison of the experimental and calculated electronic circular dichroism spectra. Cytotoxicity of the isolated compounds against four human tumor cell lines were assayed. 5 exhibited strong cytotoxicity against HCT-15, PC-3, A549 and Hela cells lines with IC50 values of 5.3, 5.6, 6.2, and 8.2 mu M, respectively. 2 showed non-concentration dependent cytotoxicity against HCT-15, PC-3, and A549 cells lines with inhibition rate of 56.1%, 55.3%, and 50.1%, respectively. 1 and 3 showed moderate cytotoxicity against four cell lines with IC50 values ranging from 12.2 +/- 5.1 to 28.6 +/- 2.5 mu M. The influence of melanin content in B16 melanoma cells of 1, 5, and 6 were tested, and they increased melanin content by 43.6%, 28.1%, and 37.0% higher than positive control 8-methoxypsoralen.