Ovarian tumors of an epithelial origin in pediatric age are uncommon and mucinous cystadenoma, borderline malignancy is extremely rare. We experienced a case of ovarian mucinous cystadenoma, borderline malignancy in a 14-year-old girl. She was admitted due to huge abdominal distension for 2 months. Her past history and family history were normal. The physical examination was normal except tense and distended abdomen. All laboratory findings were normal. On abdominal ultrasonography and CT scan, a huge cystic tumor filled with fluid and inner multiseptation was noted. Unilateral salphingo- oophorectomy was done and pathologic findings were compatible with ovarian borderline malignancy of mucionous cystadenoma. The postoperative course was uneventful.