A gastrointestinal stromal tumor (GIST) is the most common mesenchymal tumor of the gastrointestinal tract and expresses CD117, a c-kit proto-oncogene, which can be detected immunohistochemically. We reported a GIST of the rectum of a 61-year-old-woman who had visited emergency room complaining of constipation over one week. Upon rectal examination, a round hard mass was palpated. Colonoscopy showed a 7×5 cm sized protruded lesion with surface ulceration on a rectum, adjacent the anus. And abdomen computed tomography revealed the soft tissue mass compressing anterolateral wall of the rectum and these findings suggest possibility of rectal submucosal tumor such as GIST. The patient had been treated with a ultra anorectal anastomosis with loop ileostomy. Immunohistochemical studies on the surgically resected specimen showed c-kit (+) and CD34 (+). The final diagnosis was a GIST of the rectum. She was grouped into high risk and she has been given adjuvant chemotherapy with Imatinib.