In this study, we attempted to clarify the influence of the DNA repair genes RAD54 and KU70, components of the homologous recombination (HR) and non-homologous end-joining (NHEJ) pathways, respectively, on apoptosis induced by 1 Gy (low-dose) and 5 Gy (high-dose) irradiation. All experiments were performed using chicken B-lymphocyte DT40 cells and the DNA repair-deficient cell lines KU70^−/−, RAD54^−/− and KU70^−/−/RAD54^−/−. Morphological changes were detected by fluorescence methods, and the sub-G₁ fraction and the activated caspases in DT40 cells were analyzed by flow cytometry. Irradiation with 1 Gy significantly increased the level of apoptosis in cells with the defective DNA repair genes, with the maximum apoptosis occurring in double mutant cells, KU70^−/−/RAD54^−/−, demonstrating that 1 Gy is enough to induce apoptosis in DNA repair-deficient DT40 cells, and that KU70 and RAD54 must have almost the same role in low-dose radiation-induced apoptosis. After 5 Gy, fast induction of apoptosis, within 2 h, was seen in both wild-type cells and RAD54^−/− cells, indicating that functional KU70 must be important for the rescue of the cells from the induction of fast apoptosis.