A case of malignant degeneration of benign cystic teratoma of the ovary is reported. Almost all cases of malignant degeneration in the literature are squamous cell carcinomas, and other varieties or mixed malignancies are rare. But in this case, the tumor contained squamous cell carcinoma and adenocarcinoma separately. The focus of the adenocarcinoma looked like an embryonal structure of the intestine. Immunohistochemically, CEA is positive in both malignancies, but AFP is only in adenocarcinoma. These findings suggest that the pathogenesis of adenocarcinoma is different from that of squamous cell carcinoma arising in benign cystic teratoma of the ovary.