Small cell carcinoma of the ovary is rare malignancy occurring in women under 40 years of age(average, 23 years), which is associated with hypercalcemia in two thirds of cases. Its histogenesis is uncertain, but the possibilities of common epithelial, neuroendocrine, sex cord stromal and germ cell origin are suggested. All reported cases were proved to have rapid fatal course despite various therapy and 5 years suvival rate was only 10%. We report one case of a 20-year old woman with primary small cell carcinoma of the left ovary. The ovary was markedly enlarged and completely replaced by a mass, measuring 21x16x8 cm. Microscopic examination revealed dimorphic population of small and large malignant cell producing immature follicle-like structure which is characteristic of small cell carcinoma of the ovary. These pathological findings were similar to those of granulosa cell tumor, which is required to make differential diagnosis from small cell carcinoma. Immunohistochemical stains for cytokeratin and vimentin were positive, but those for S-100 protein and NSE were negative. One month after the initial operation, the tumor has recurred and the second and the second palliative operation followed by 3 cycles of chemotherapy was done. The patient showed disseminated metastasis at present time.