Four new meroterpenoids, arnebinols A-D (1-4), and one new prenylated naphthoquinone, 5,8-O-dimethyl-11-deoxyalkannin (5), together with seven known meroterpenoids (6-12) were isolated from the roots of Arnebia euchroma. The structures of the isolated compounds were elucidated unambiguously by spectroscopic data analysis, as well as X-ray-single crystal diffraction analysis. Arnebinol A (1) and B (2) are rare meroterpenoids possessing a 6/10/5 tricyclic ring system. Compounds 1-12 were evaluated for their cytotoxicities against MG-63 and SNU387 human cancer cell lines. Compound 5 exhibited the most potent activity with IC50 values of 2.69 mu M and 6.08 mu M, respectively.