Neoplasms showing both carcinomatous and sarcomatous features are well established, and have been reported in practically every organ where carcinoma can occur. But the difference in terminology and difficulties in histopathologic interpretation have hampered adequate understanding of these neoplasms. We report a case of carcinosarcoma in the recurrent form of serous cystadenocarcinoma of the ovary. The patient was a 64-year-old female who underwent total hysterectomy and bilateral salpingoophorectomy three years ago, due to perforated papillary serous cysta denocarcinoma of the right ovary. Recurrent pelvic masses contained both carcinomatous and sarcomatous components. Morphological transition between carcinomatous and sarcomatous components, and epithelial characteristics in the sarcomatous component by immunohistochemistry were recognized. We postulate the histopathogenetic mechanism based on the phenotypic conversion of carcinoma into sarcoma in the carcinosarcoma.