Background: Radiotherapy remains a mainstream treatment for patients with glioma. Yet intrinsic radioresistance has largely compromised the efficacy of the treatment. Increasing concerns have been raised that overexpression of the Nrf2, along with a hypoxic tumor microenvironment, may have contributed to the deterioration of radiotherapy in tumors. So, this study investigated the role of Nrf2 in the radiation therapy of glioma cells in hypoxia. Methods: To determine the expression levels of Nrf2 and HIF-1 alpha, surgical mastectomy specimens from patients with glioma in our institute were analyzed by immunohistochemical staining. Glioblastoma multiforme (GBM) cell lines U251 and U87 with Nrf2 knocked down were produced by transfection with lentiviral particles. Cell lines were treated with ionizing radiation in hypoxia in vitro, with expression and activity of Nrf2 examined by polymerase chain reaction and western blot. Reactive oxygen species (ROS) generation and cell apoptosis analysis were analyzed by flow cytometry.Results: Nrf2 and its downstream pathway were upregulated in surgical specimens after radiotherapy, verified by GBM cell lines treated with in vitro ionizing radiation in hypoxia. Furthermore, knockdown of Nrf2 could induce the ROS generation and cell apoptosis levels after radiation.Conclusions: Downregulation of Nrf2 could sensitize the lethal effect on GBM cells in vitro by enhancing oxidative stress and apoptosis in hypoxia.