A 5-year-old HOLSTEIN COW was referred to the Veterinary Medicine Hospital, Urmia University, Urmia, Iran, with abnormal estrous cycle. At rectal palpation, the unilateral ovary enlargement was detected. On transrectal ultrasonography view, the left ovary had uniformly hyperechogenic areas. The affected ovary was removed by ovariectomy and sent for histopathological examination. Histopathological evaluation revealed fibroblastic cells producing collagen fibers and theca cells containing lipids. Based on histopathological features, diagnosis of fibrothecoma was confirmed. This case reports an extremely rare fibrothecoma in COW.