Uterine smooth muscle tumor is very rare in laboratory rats and, there has been no report in the wild rodents. Among a total of 400 wild rats captured in Gyeonggi, Gangwon, and Chungbuk provinces of Korea in 2007, 2010, and 2011, we found a uterine spindle cell tumor, diagnosed as smooth muscle cell origin based on differential features of histology and immunohistochemistry. Its incidence was very low, like in the laboratory rats, as under 0.5% for female. Considering generally applied histological and cellular criteria, this case was difficult in differential diagnosis between benign and malignant. Ki-67 labeling index was therefore further investigated, and it ranged from 26.4 to 37.6% in the 10 different areas, representing an average of 32.9±0.05%. The Ki-67 labeling index of neoplastic cells near the necrotic area was recorded as 83.5%. According to such high Ki-67 labeling index, it was more likely a malignant leiomyosarcoma, assenting to the previous proposal that Ki-67 labeling index is a significant criterion to differentiate between malignant and benign in the smooth muscle tumors.