Licorice (Glycyrrhiza glabra L.) is one of the most widely used plants worldwide for its various pharmacological activities. The aim of this study was to isolate the potential cytotoxic secondary metabolites from the MeOH extract prepared from the roots of Glycyrrhiza glabra through bioactivity-guided isolation procedure and to elucidate their mechanisms of action. The crude MeOH extract as well as CHCl3 and EtOAc subextracts significantly inhibited cell proliferation on hepatocelullar (Huh7), breast (MCF7) and colorectal (HCT116) cancer cell lines with IC50 values in the range of 5.6 to 33.6 mu g/mL. Chromatographic seperations of the CHCl3 and EtOAc subextracts yielded 13 secondary metabolites. Structures were characterized based on NMR and MS data. Amongst isolates, glabridin (1), 4'-O-methylglabridin (2), beta-amyrin (3), kanzonol U (4), glabrene (7) and tet-rahydroxymethoxychalcone (10) were established to be responsible for in vitro cytotoxicity of G. glabra, exerting the best activity particularly against Huh7 cells. Further mechanistic studies demonstrated that 2 and 7 induced caspase-dependent apoptosis by increasing cytochrome C release and subsequently cleaved caspase-9 level in Huh7 cells. Moreover, both compounds decreased pRb and p21 levels and thus induced the accumulation of Huh-7 cells in subG(1) and G(2)/M phases. Compound 10 which displayed the most potent activity in Hoechst staining and cell cycle assays through G(2)/M arrest, caused cell death by apoptosis in Huh7 cells.