Background/Aim: Using nationwide data from South Korea, we analyzed the prevalence of mental disorders among patients with nasopharyngeal carcinoma (NPC) who received radiotherapy (RT). Patients and Methods: Data were collected between January 2010 and December 2014, and 1,819 patients diagnosed with NPC who underwent RT were identified. We calculated the prevalence of the five most commonly diagnosed mental disorders and their incidences according to a time sequence based on the time of RT. Results: Among 1,819 patients, 144 (7.9%) were diagnosed at least once with a mental disorder between one year before the start of RT and the last follow-up. Based on the first diagnosis, 51 (35.4%) patients experienced anxiety, and 46 (31.9%) suffered from depression. The frequency of mental disorders increased up to the beginning of RT and then declined. Before the start of RT, anxiety was most frequent, and depression occurred after the start of RT. The cumulative incidence of mental disorders in the older age group tended to be higher than that of the younger group (≥54 vs. <54 years, log-rank p=0.052). Conclusion: The proportion of mental disorders differed before and after the start of RT in patients with NPC. At the beginning of RT, mental disorders emerged most often during management. Early screenings and interventions for mental disorders were able to improve quality of life (QOL).