A random bioassay revealed that the EtOH extract and EtOAc fraction of Artemisia dubia Wall. (Asteraceae) exhibited cytotoxic activity against HepG2 cells with inhibitory ratios of 57.1% and 84.2% at a concentration of 100.0 μg/mL. Bio-guided isolation combined by LC-MS-IT-TOF analyses of the active fractions led to the isolation of 20 previously undescribed guaiane-type sesquiterpenoid dimers named artemidubolides A-T (1-20). Their structures and the absolute configurations were determined by comprehensive spectral analyses, comparison of the experimental and calculated ECD spectra, and seven compounds (artemidubolides A, B, D, F, K, O and R) were confirmed unequivocally by single crystal X-ray diffraction analysis. Structurally, artemidubolides A-Q were [4 + 2] Diels-Alder adducts of two monomeric guaianolides, and artemidubolides R-T were linked though an ester bond. All the isolated compounds were evaluated for their hepatomatic cytotoxicity against HepG2, Huh7, and SK-Hep-1 cell lines to demonstrate that 18 compounds exhibited obvious cytotoxicity against three tested hepatoma cell lines with IC50 values in the range of 5.4-87.6 μM. Importantly, artemidubolides B, D, and M exhibited hepatoma cytotoxicity with IC50 values of 5.4, 5.7, and 9.7 (HepG2), 8.2, 4.3, and 12.2 (Huh7), and 13.4, 8.4, and 12.9 μM (SK-Hep-1), respectively. Mechanism investigation in HepG2 cells suggested the most active artemidubolide D dose-dependently inhibited cell migration and invasion, induced G1/M cell cycle arrest by down-regulating proteins CDK4, CDK6 and CyclinD1 and up-regulating the level of protein P21; and induced apoptosis by down-regulated of PARP-1 and BCL-2 expression and up-regulating Bax and cleaved PARP-1 levels.