Retroperitoneal fibrosis is a rare disease characterized by the formation of dense plaque of fibrous tissue covering the retroperitoneal structures. This disease is commonly presented as ureteral obstruction, but the involvement of duodenum is rare. We report a case of retroperitoneal fibrosis which was complicated with duodenal stenosis and was successfully treated with corticosteroids. A 58-yr-old man, who had history of aorto-iliac bypass graft due to arteriosclerosis obliterans with infrarenal aortic occlusion was admitted to the hospital with abdominal pain and a mass. Abdominal CT scan revealed the periaortic soft tissue mass encircling grafted aorta and stenosis of duodenal third portion. Retroperitoneal fibrosis with duodenal stenosis was diagnosed and prednisolone therapy was initiated. Follow-up CT scan showed that the patient responded to prednisolone therapy with eased pain, shrinking periaortic mass, and reduced duodenal stenosis.