Purpose : To evaluate the effect of all-trans-retinoic acid on radiosensitivity and radiation-induced apoptosis in NHOK, HEp-2 and FaDu cell lines. Material and Methods: We measured the changes in survival fraction at 2 Gy (SF2), α and β after treatment of retinoic acid (1μM) prior to irradiation with doses of 2, 4, 6 and 10 Gy and correlated the radiosensitizing effect of retinoic acid with them. Also, apoptosis induction was assayed with the flow cytometry on days 1,2, 3, 4 and 5 after irradiation (2, 10 and 20 Gy) combined with retinoic acid. Results and Conclusion: SF2 values for NHOK, HEp-2 and FaDu cell lines were 0.54, 0.64 and 0.41, respectively and the cell line of FaDu was the most radiosensitive. For cell lines of NHOK and HEp-2, pretreatment of cells with retinoic acid resulted in a significant decrease of the SF2 values. The α/β ratios of x-ray survival curve were 8.714 (NHOK), 4.098 (HEp-2) and 11.79 (FaDu). The α/β ratio for NHOK decreased on pretreatment with retinoic acid, whereas those for HEp-2 and FaDu increased. Radiation induced apoptosis in all cell lines but, retinoic acid did not affect the apoptosis.