The development of mucinous cystadenocarcinoma of low malignant potential in a mature cystic teratoma is rare. We report a 36-year-old single female presented with abdominal distension and was found to have a huge pelvic/abdominal mass. Ultrasound revealed a huge cystic ovarian mass with no ascites. Laparotomy and left oophorectomy was performed to the mass. Histology revealed mucinous cystadenocarcinoma of low malignant potential in a mature cystic teratoma.