Introduction: Lymphomas comprise a diverse group of neoplasms derived from B cells, T cells, or NK cells. Although GI tract is a current site for the secondry spread of non-hodgkin lymphoma, it's involvment as the primary site is less common noticeably showing only 10% -15%. The more lymphoid tissue is in parts of small intestine, the more incidence of lymphoma is depicted; thus, as expected, ileocecal region is the most frequent site and the duodenum remains as the most infrequent site. Case Report: An 18-year-old woman with no pervious history of any disease was admitted to the emergency room of Shohaday-eTajrish hospital that is a tertiary hospital in Tehran, Iran. The patient presented with weakness, nausea, and retractile vomiting. The CT scan was performed, showing that stomach and proximal of duodenum were dilated significantly and wall thickening of duodenum, 2nd and 3rd portions, was obvious. No pathological lymph node or invasion to adjacent organs was seen. The patient was explored; operation findings were dilatation of stomach and duodenum; thickening of duodenojejunal junction with 10 cm extension was mentioned. Biopsied tissue during operation showed diffuse large B cell lymphoma. Conclusions: Duodenal lymphoma is infrequent and it is rare to represent with obstruction. Thus, if there is any clinical indication and no definite diagnosis in such patient, biopsy and histopathological investigation is necessary after radiological studies.