The mature cystic teratoma is the most common ovarian neoplasm arising in young women. The vast majority of them is benign, however malignant transformation has been reported to occur in 1~2% of these lesions. The malignant neoplasms arising in mature cystic teratoma are overwhelmingly squamous cell carcinomas. Adenocarcinoma occurs with less frequency. The prognosis does not appear to be improved by range of surgery but depends mainly on the tissues of the primary tumor and the peritoneal deposits. We experienced two cases of squamous cell carcinoma, two cases of adenocarcinoma and one case of papillary cell carcinoma arising in mature cystic teratoma of the ovary and report these cases with a brief review of literature.