Exposure of cells to ionizing radiation leads to formation of reactive oxygen species (ROS) that are associated with radiation-induced cytotoxicity. The antioxidant enzymes manganese superoxide dismutase (MnSOD), copper-zinc superoxide dismutase (CuZnSOD), and glutathione peroxidase (GPx), are key intracellular antioxidants in the metabolism of ROS. We investigated the potential role of the three antioxidant enzymes in radioresistance by analyzing cell morphology, cell viability, and cellular radioresistance in stably transfected CHO cell lines overexpressing MnSOD, CuZnSOD, and GPx, respectively. We found that overexpression of human MnSOD substantially protected cells from ionizing radiation injury. Relative to MnSOD, GPx could slightly reduce the radiation sensitivity while the radioresistance in CuZnSOD expressing cells did not change significantly. The results suggested that MnSOD may play a central role in protecting cells against reactive oxygen species injury during ionizing radiation exposure among MnSOD, CuZnSOD, and GPx.