A 73-year-old woman was admitted for closer investigation into an abdominal tumor. Abdominal ultra-sonography, CT, and magnetic resonance imaging showed a discrete abdominal tumor. Especially on MRI, the tumor appeared as a relatively ill-defined whorled soft-tissue thickening within the mesenteric fat, causing angulation or speculation of adjacent bowel mesentery. Surgery was performed under a presumptive diagnosis of a mesojejunum desmoid. The tumor was invading the transverse mesocolon and mesojejunum as well as the third portion of the duodenum and the middle colic artery, and it was close to but apart from the superior mesenteric artery. The tumor was resected, including removal of part of the duodenum and transverse colon. The specimen contained a white solid tumor measuring 14×12×8cm. Pathologic examination showed differentiated fibroblasts and copious collagen fibers. The tumor was negative for CD34, c-kit, S-100, and α-SMA, but positive for β-catenin. On the basis of these findings, we established a diagnosis of mesenteric desmoid tumor of the small intestine. The patient has been followed postoperatively for 2 years, no sign of recurrence, to date.