Objective: To evaluate the risk of unexpected uterine smooth cell tumor of uncertain malignant potential (STUMP) or sarcoma during surgical treatment of myoma in a single institution. Methods: Data were collected retrospectively of subjects who were pathologically diagnosed with leiomyoma or its variants, STUMP or other premalignant smooth muscle cell tumors, and uterine sarcoma between July 2014 and June 2017. Results: A total of 3785 women were included; 2824 laparoscopic procedures (74.6%) were performed, and electronic power morcellator was used in 1636 patients (43.2%). Sixteen (0.42%) women were diagnosed with STUMP and 14 (0.37%) were diagnosed with uterine sarcoma. The incidence of unexpected STUMP or sarcoma was 0.61% (23 in 3785 women); unexpected STUMP was 0.34% (13 in 3785), unexpected LMS was 0.08% (3 in 3785), and total unexpected uterine sarcoma was 0.26% (10 in 3785). The rate of unintended morcellation of the STUMP was as high as 0.26% (10 in 3785), however, in case of uterine sarcoma was 0.05% (2 in 3785). Conclusion: Overall, the risk of unintended morcellation of sarcoma seemed to be very low; however, the risk in cases of STUMP was relatively higher than in sarcoma. In order to reduce the unintended dissemination of tumor, the patients of suspected cases should be provided with an adequate informed about the selectable options and following risks of the treatment methods while established preoperative screening methods for STUMP and sarcoma.