Abstract: Objective: To investigate the mechanism behind the anti -proliferative effect of Kanglaite (KLT) on the hepatocellular carcinoma cell line HepG2. Methods: The hepatocellular carcinoma cell line HepG2 was treated with KLT at doses of 15μl/ml (the high concentration group), 10μ l/ ml (the moderate concentration group) or 5μl/ml (the low concentration group). The expressions of cyclin D1 and cyclin E proteins were detected by immunohistochemistry, and the MTT assay was used to observe the inhibitory effects of KLT on cell growth. The cell-cycle distribution was assayed by flow cytometry. Results: a) KLT produced an obvious time- and -dose-dependent inhibitory effect on the HepG2 cells. b) The ratio of G1 phase cells was increased to 75.8%, 70.1% and 66.5% for the low, moderate and high concentration groups, showing a ssignificant difference from the untreated controlse (P<0.05). The ratio of S-phase cells was decreased to 13.2% and 20% for the high and the moderate concentration group, respectively, so showing a significant difference (P<0.05). c) The immunohistochemistry showed that in the low, oderate and high concentration groups the level of cyclin D1 protein decreased 26.3%, 35.1% and 45.6%, respectively, from the level found in untreated cells, . The level of cyclin E protein decreased 18.8%, 27.1% and 33.3%, respectively. These decreased levels showed a significant difference in comparison with the controls (P<0.05). Conclusion: KLT produced obvious time- and dose-dependent inhibitory effects on HepG2 cells. The mechanism for arresting HepG2 cells bywith KLT involves inhibition of the expression of cyclin D1 and cyclin E proteins which, prevents the cells from progressing into S phase.