Struma Ovarii was rare and was defined as ovarian teratoma, composed solely or predominantly of thyroid tissue, or with definite evidence of functionning struma ovarii manifested as hyperthyroidism. Most cases of struma ovarii was diagnosed after operative resection, so radionuclide study was incomplete. This report concerns with 49 year old women who had malignant struma ovarii with clinical manifestation of hyperthyroidism. I131 thyroid function study confirmed it and could delineate the tumor in the pelvic cavity. Surgical resection was not possible. So we administered internally large dose of I131.