Leiomyosarcoma is a rare tumor that originates in the smooth muscle, usually in the gastrointestinal tract, the retroperitoneum and the genitourinary tract. The omentum has been rarely reported as a primary site of occurrence with leiomyosarcoma. A 72 year-old woman, with palpable mass on the left upper quadrant of abdomen and weight loss, was admitted to St. Paul's Hospital. Abdominal CT scan showed a 12 x 8 x 8 cm-sized cystic mass in the left upper quadrant of the abdomen. Endoscopic ultrasonography showed a large cystic mass between the stomach and the liver, which was 1.6 cm length in wall thickness. Laparotomy and resection of the mass was performed. A 12 x 8 x 8 cm-sized mass, originated from the lesser omentum, was discovered near the lesser curvature of the stomach. Microscopic examination revealed spindle-shaped cells with 7-8 mitoses per high power field. She was diagnosed as primary leiomyosarcoma originated from the lesser omentum.