We report a rare case of gastrointestinal stromal tumor of rectum. A 57-year-old-woman was admitted to our hospital complaining of genital bleeding. MRI showed the presence of an 11.2×8.7cm cystic tumor in pelvis. The border is unclear between the tumor and vagina and rectum. We could not get the diagnosis with biopsy and performed partial resection of vagina and rectal amptation considering the tumor as a vaginal tumor. Histologic examination revealed fascicular proliferation of spindle-shaped tumor cells developed from rectum. Immunohistochemical testing was c-ki（t +）, CD 34（+）, smooth muscle actin（‐）, desmin（‐）and S‐100 protein（‐）. The tumor was diagnosed as GIST of rectum. Post-operative course was uneventful and no evidence of recurrence is detected 13 months after the operation. We could not get the preoperative diagnosis and performed the operation as a vaginal tumor, however rectal GIST should be considered when detect a pelvic tumor
