Malignant transformation of mature cystic teratoma is rare and the vast majority are squamous cell carcinoma, but neuroblastomas are extremely rare. We report a case of neuroblastoma arising in a mature cystic teratoma of the retroperitoneum. In a 24-year-old woman with intermittent abdominal pain, sonogram and CT scan show a multiloculated cystic mass with calcification, fat, septum, and small solid component in the left retroperitoneal space. Complete surgical resection of the cystic mass was done and a small neuroblastoma in the septum of the cystic mass was incidentally detected at histopathologic examination.