We reported a case of 35-year old man who was diagnosed primary malignant lymphoma in duodenum confirmed by post-operation pathologic diagnosis. He complained of general weakness and epigastric pain. The endoscopy revealed marked duodenitis with necrosis and bleeding of the bulb and the 2nd portion of duodenum. The result of endoscopic biopsy was chronic inflammation with some atypism. The massive upper GI bleeding was developed and Whipples operation was done. The pathologic examination of specimens showed diffuse, well differentiated, lymphocytic malignant lymphoma.