Primary small cell carcinoma of the vagina is quite rare, and a standard treatment has not been established yet. Herein, we report a case of an 81-year-old woman who was diagnosed with a vaginal tumor without continuity with the uterine cervix. Histopathological diagnosis indicated alveolar solid growth of nuclear chromatin-rich atypical cells with a high N/C ratio and a partially recognized rosette-like structure, suggesting a differentiated neuroendocrine system. Chromogranin A and synaptophysin were positive. Stage I vaginal small cell carcinoma localized to the vagina was diagnosed. The tumor disappeared by radiation monotherapy with external beam irradiation and endocavitary irradiation. The patient remains alive without any disease 1 year and 8 months after the treatment, suggesting the efficacy of radiotherapy in small cell carcinoma of the vagina.