A patient, a 63 year-old-man, was admitted suffering from discomfort in the left abdominal area; this proved to be a case of leiomyosarcoma of the mesentery. An upper gastrointestinal series revealed stenosis in the 3rd portion of the duodenum, and shift of the entire intestine to the right side. Computed tomography showed a giant mass lesion with a central necrosis. Selective arterial angiography showed a heterogeneous tumor stain with several feeders and drainage veins. A partial resection of the intestine around the Treitz's ligament and left hemicolectomy was required due to the tumor's invasion of the intestinal wall and transverse colon. The operation was successfully performed supported by the angiographic findings. The resected tumor was 23 cm in diameter, 2330 g in weight, and was filled with blood. The histological diagnosis was leiomyosarcoma of the mesentery. The patient has been doing well during the 6 months postoperative period.