Ovarian tumors are common neoplasm in women. It accounts for about 5% of all gynecological cancers in India. Borderline Mucinous cystadenoma ovary is extremely rare in children that only few cases are reported. We presented a case of Borderline Mucinous cystadenoma ovary for its rare occurrence in 14 years old girl, presented with symptoms of abdominal distension since last 6 months. Examination showed a single 0, smooth, cystic mass of 30 weeks pregnant uterus. USG examination showed huge cystic mass arising from right sided ovary. Oophorectomy was performed and send for histopathology examination. Tumor was measures 21x21x7 cm and of 5.2kg weight. Histopathology was diagnostic tool. It was diagnosed as case of Borderline Mucinous cystadenoma ovary. Recurrence was not seen postoperatively. Borderline mucinous cystadenoma have favourable outcome in adloscent girls and salpingo oophorectomy or oophorectomy appear appropriate to preserve fertility.