It is difficult to distinguish an islet tumor originating from heterotopic pancreas tissue from the other submucosal tumors. Although the malignant transformation of a heterotopic pancreas, including islet cell tumor, is extremely rare, it remains an important consideration in the differential diagnosis of duodenal submucosal masses. We have demonstrated the radiologic appearance and the clinical-pathologic findings of a highlighted, rare case of islet cell tumor arising from a heterotopic pancreas in the duodenal wall.