We report an incidental case of uterus-like mass located in the right ovary. A mass, measuring 7 mm in its largest dimension was noted in the right ovary of a 54-year-old women. Histologically, the lesion was resembling a miniature uterus with a cavity lined by a tubal-like epithelium, and surrounded with circumscribed smooth muscle bundles. Since 1981, 8 cases of uterus-like mass arising from the ovaries have been reported and their pathogenetic mechanism has been explained by two basic theories; congenital anomaly and metaplasia. The present case was incidentally found. The uterus was unicornus secondary to fusion defect of müllerian duct during embriogenesis supporting the congenital anomaly theory.