Leiomyosarcoma is defined as a malignant smooth muscle tumor or a tumor that arises from smooth muscle and it is found as firm. white and lobulated mass. Canine uterine Ieiomyosa- rcomas are rarely associated with clinical signs. We surveyed(one case which was treated at veter- inary medical leaching hospital of Seoul National University. Through this report we studied the meshed fur diagnosis of uterine leiomyosarcoma by symptom, hematological method and ultrahonog- raphy. To determine uterine leiomysarcoma is difficult due to similarity of clinical signs to pyometra. Especially symptoms could not be noticed even by dogs owners until dogs have an elevation of the number of white blood cells. We conclude ultrasonographic observation and experimental laparotomy are of great value to diagnose the uterine liomyosarcoma.