Gliomatosis cerebri (GC) is a rare disease loosely defined as a diffusely infiltrating glioma involving extensive areas of the brain. The prognosis is poor and no definite treatment has proven effective for GC. Little information exists regarding the role of radiation therapy (RT) for GC, but some researchers have suggested that it is a good choice of treatment from their limited experience. In this report, we present a case with imaging and histological diagnosis of GC and demonstrate the treatment results of RT. The patient was a 39-year-old woman with progressive symptoms of dizziness, unsteady gait, headache, vomiting, and consciousness disturbance for 6 months. She received a series of radiographic examinations and surgical interventions for diagnosis. The definite diagnosis of GC was made by a combination of magnetic resonance imaging (MRI) findings and histological examinations. Forty Gray (Gy) of whole brain irradiation followed by 14 Gy reduced-field boosts were given to her. The MRI, following treatment, showed regressive changes, and clinical symptoms were slightly improved. The patient survived 19 months after the diagnosis, which is longer than the average survival time of patients without treatment.