Background. Development of neuroepithelial tumors from mature cystic teratoma is very rare. We present a case of oligodendroglioma developing inside mature cystic teratoma. Case. Eighteen-years-old female, right adnexal mass with solid and cystic areas was detected. Sections showed all three germ layers. Also, a tumoral lesion was observed in a glial fibrillary matrix. Tumor was composed of monotonous, uniform cells which have oval-round nucleus, perinuclear halo, and indistinct cytoplasm. GFAP, EGFR, P53 were positive. Conclusions. We diagnosed oligodendroglioma arising from mature cystic teratoma. There was no recurrence at the end of 13 months followup. The number of cases which have been reported in the literature is only a few.