Ionizing radiation induces more cell death under normoxic conditions than under hypoxic conditions. This phenomenon, which is known as the oxygen enhancement effect, occurs primarily because ionizing radiation causes more DNA lesions in the presence of oxygen than in its absence. However, the roles these lesions play in terms of cell survival and chromosome damage have not been fully characterized. We exposed a panel of chicken DT40 mutant cells to ionizing radiation to categorize the type of lesion induced and the DNA-repair pathway involved under both normoxic and hypoxic conditions. Among the mutant panel, RAD54^–/–/KU70^–/– cells exhibited the greatest radiosensitivity, which was found to be significantly higher under normoxic conditions. This indicates that double-strand breaks (DSBs) were the major cause of cell death and that ionizing radiation induces more DSBs under normoxic condition. Interestingly, the sensitivity of the REV3^–/– cells increased under hypoxic conditions. Indeed, the REV3^–/– mutant exhibited a greater number of chromosomal aberrations under hypoxic conditions than under normoxic conditions. These results suggest that the Rev3-mediated translesion-synthesis pathway is more critical for cellular tolerance to ionizing radiation in hypoxic cells than in normoxic cells, and that more chemically modified DNA might be induced under hypoxic conditions. In this study, we identify a previously unappreciated radiation-induced pattern of DNA damage under hypoxic conditions.