Gastrointestinal stromal tumor (GIST) represents the most common kind of mesenchymal tumor that arises from the alimentary tract. GIST is currently defined as a gastrointestinal tract mesenchymal tumor showing CD117 (c-kit protein) positivity at immunohistochemistry. Throughout the whole length of the gastrointestinal tract, GIST arises most commonly from the stomach followed by the small intestine, the colorectum, and the esophagus. Only 3%-5% of GISTs occur in the duodenum, and especially, if GIST arises from the C loop of the duodenum, it can be difficult to differentiate from the pancreas head mass because of its anatomical proximity. Here, we report a case of duodenal GIST, which was assessed as a pancreatic head tumor preoperatively.