International audience ; Malignant struma ovarii is a rare tumor. This is a report of case of 45 year old female patient who was coming for a routine control at gynecology serv ice of Saint Pierre clinic of Ottignies. During the check-visit, a left -attached pelvic mass was obser ved. The mass was confirmed at the abdominal ultrasound scan and magnetic resonance imaging scan . It was found localized at the left ovary. Follicular variant of papillary carcinoma type of m alignant struma ovarii was diagnosed after a total radical hysterectomy and an omentectomy were done. Among two critical treatments that are classically proposed in such cases, a thyroidectomy followed by a 131 iodine radiotherapeutic ablation was found to be effective. Thyroxin suppre ssive treatments followed by biological and clinical assessments were applied. They were found to be effective since the patient had a positive evolution.