Duodenal gastrointestinal stromal tumors (GISTs) are uncommon. Tumors arising from the first and the second part of the duodenum (DI and DII, respectively) can be wrongly diagnosed as pancreatic mass. We present a case of a 59-year-old woman who came with abdominal pain and severe upper gastrointestinal bleeding (hemoglobin 3.5 g÷dL). A solid, heterogeneously enhancing neoplasm in the head of the pancreas was revealed preoperatively by an abdominal computed tomography scan. A diagnosis of GIST was suggested. On exploratory laparotomy, there was a large mass which appeared to be originating from duodenum or from head of pancreas. Intraoperative histopathological diagnosis was GIST. Histopathology showed spindle cell tumor with cytoplasmic eosinophilia and foci of necrosis. The mitotic count was less than 5÷50 high power fields (HPFs). Tumor was involving duodenal muscularis propria, with no infiltration in the duodenal epithelial layer and the pancreas. Immunohistochemical study revealed positive staining for CD117. The tumor was finally diagnosed as GIST arising from the duodenal wall, growing exophytically and attached to the common bile duct and pancreas, without infiltrating the pancreas. Duodenal gastrointestinal stromal tumors can grow exophytically into a large mass and involve the pancreas without infiltrating microscopically and present as pancreatic head mass.