Malignant transformations of mature cystic teratomas (MCT) are extremely rare and most of them are squamous cell carcinomas (SCC). Therefore no effective postoperative treatment has been established. In this article, we report two cases in which radiotherapy was effective for SCC arising from MCT. Case 1 was a 64-year-old woman with stage IIA of this tumor. After primary surgery, chemotherapy and interval debulking surgery were performed. She received radiotherapy for relapsed tumors, and has been well for 36 months since the initial diagnosis. Case 2 was a 37-year-old woman with stage IIB of this tumor. After primary debulking surgery, she received chemoradiotherapy for a residual tumor and has been well for 27 months since the surgery. Although there is no established therapy, radiotherapy or concurrent chemoradiotherapy might have beneficial effects on this tumor, similarly to those on SCC from other tissue.