Malignant struma ovarii is a rare form of ovarian carcinoma. Since it is often seen as a nonspecific complex mass, its preoperative diagnosis is difficult. The MR imaging findings of benign struma ovarii have been reported, but no published account has described the malignant form. In this case, MR images depicted multiple cysts with well enhanced solid portions in the left ovary, and the typical apperarance of mature cystic teratoma in the right ovary. We describe a case of malignant struma ovarii associated with mature cystic teratoma of the contralateral ovary.