Malignant transformation in a mature cystic teratoma is rare and sarcomatous change is even rarer. We report a case of 57 year old post-menopausal woman who presented with tender abdominal mass and fever. She underwent Total Abdominal Hysterectomy with Bilateral Salpingo-oophorectomy. Left ovary showed a mature cystic teratoma with solid areas. Histopathology of these solid areas showed a high grade spindle cell sarcoma which exhibited Vimentin and SMA positivity on Immunohistochemistry and was thus diagnosed as leiomyosarcoma. In most instances malignant transformation cannot be diagnosed pre-operatively. Most of these tumors are considered resistant to radiation and systemic chemotherapy and carry a poor prognosis.