Farnesyltransferase inhibitor (FTI) acts on ras, which can ultimately enhance radiosensitivity. The objective of this study was to explore whether FTI could potentiate the antitumor efficacy of radiation in vivo, particularly in radio-resistant hepatocarcinomas (HCa-I) syngeneic to C3H/HeJ mice. The presence of ras mutations was examined by PCR and DNA sequencing. C3H/HeJ mice, bearing HCa-I, were treated with FTI, LB42907, and 25 Gy radiation. FTI was orally administered, 60 mg/kg, twice daily for 30 days. The expression of regulating molecules was analyzed by Western blotting for p53, p21(WAF1/CIP1), and the Bcl-2 family, such as Bcl-2, Bax, and Bcl-X(L/s). In HCa-I, no ras mutations were detected. Downregulation of ras by FTI was most prominent at 4 h after treatment. In a tumor growth delay assay, FTI increased the effect of the tumor's radioresponse, with an enhancement factor of 1.32. Combined irradiation and FTI increased radiation-induced apoptosis; the peak apoptotic index was 3.6% with irradiation alone and with the drug alone but 7.1% in the combined treatment group. The analysis of apoptosis-regulating molecules by Western blotting showed upregulation of p53 and p21(WAF1/CIP1) in the combined treatment group compared with those in either of the single treatment groups, but the Bcl-2 family remained unchanged. FTI, in combination with radiation therapy, may have potential benefits in cancer treatment even if there are no ras mutations. FTI could inhibit ras activity but may also affect any protein that requires farnesylation for its activity.