Primary malignant struma ovarii is a rare form of ovarian germ cell tumors. Germ cell tumors constitute 15-20% of ovarian tumors and the majority of them are mature cystic teratomas. While 5-15% of teratomas contain small foci of thyroid tissue, struma ovarii is diagnosed when thyroid tissue is the predominant element. Approximately 5% of struma ovarii show malignant transformation. Due to its rarity, there has been some controversy about the diagnosis and treatment, and prognosis is difficult to evaluate. Recently we have experienced a case of malignant struma ovarii of the ovary, so we report this case with a brief review of literature.