Flavonoids are a broadly distributed class of plant pigments, universally present in vascular plants and responsible for much of the coloring in nature. They are strong antioxidants that occur naturally in foods and can inhibit carcinogenesis in rodents. In this study, we examined acacetin (5,7-dihydroxy-4'-methoxyflavone), a flavonoid compound, for its effect on proliferation in a human liver cancer cell line, Hep G2. The results showed that acacetin inhibited the proliferation of Hep G2 by inducing apoptosis and blocking cell cycle progression in the G1 phase. Enzyme-linked immunosorbent assay showed that acacetin significantly increased the expression of p53 and p21/WAF1 protein, contributing to cell cycle arrest. An enhancement in Fas/APO-1 and its two form ligands, membrane-bound Fas ligand and soluble Fas ligand, as well as Bax protein, was responsible for the apoptotic effect induced by acacetin. Taken together, our study suggests that the induction of p53 and activity of the Fas/Fas ligand apoptotic system may participate in the antiproliferative activity of acacetin in Hep G2 cells.