Eight flavonoid C-glycosides, including three new analogues, have been isolated from leaf and root methanolic extracts of Petrorhagia velutina, a Mediterranean herbaceous plant. The antiproliferative activity against human hepatoblastoma cancer cell line HepG2 has been analyzed by the MTT (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide) test. Isoorientin (4) significantly reduces the proliferation of HepG2 cells as determined by the complete conversion of the tetrazolium probe into formazan after 48 h of exposure.