Here, we report that a loss or decrease of RUNX3 expression was found in 73 cases of HCCs as compared with that in normal liver tissues (p < 0.001). Various human HCC cell lines also exhibited loss or decrease of RUNX3 expression. The introduction of RUNX3 by an adenovirus vector into HCC cell lines which had decreased expressions of RUNX3 inhibited cell proliferation and cell cycle progression, decreased anchorage-independent growth, and inhibited tumorigenesis in nude mice. Exogenous expression of RUNX3 sensitized HCC cells to cytotoxic drugs and to apoptosis induced by chemotherapeutic drug adriamycin in vitro. Ectopic expression of RUNX3 in HCC cells enhanced caspase-8 and decreased Bcl-2 expression. Treatment of nude mice bearing subcutaneously established HCC tumors with a combination of an adenovirus expressing RUNX3 and adriamycin completely suppressed tumor growth. In conclusion, overexpression of RUNX3 might be a promising candidate as a treatment for HCC that would increase sensitivity to chemotherapeutic drugs.