Malignant struma ovarii is a rare form of ovarian germ cell tumors. Because of its rarity, diagnosis and management of the tumor has not been clearly defined. A 52-year-old female with follicular variant of papillary carcinoma arising in struma ovarii is presented. There was no evidence of clinical malignancy or metastases. However on the basis of histopathologic findings, the case was diagnosed as malignant struma ovarii. It was accepted as stage Ia tumor and no other treatment was considered after radical excision. Due to its fairly good prognosis, surgical excision has been the preferred treatment for those who do not have disseminated disease. Nevertheless, long-term follow-up is necessary to determine the course of the disease.