A 69-year old man was found a mass becoming larger in abdominal computed tomography. The mass consisted of intermingling solid and cystic component was located below the liver. Abdominal angiography showed tumor staining supplied from right gastroepiploic artery. We considered the mass cystadenoma, lymphangioma, cystic mesothelioma, or gastrointestinal stromal tumor (GIST) preoperatively, and then surgical resection was performed. The tumor was found localized in the greater omentum. Pathological examination showed the tumor composed of proliferation of atypical sort spindle cells and tumor cells were immunohistochemically positive for C-KIT and CD34, identifying the tumor as a primary GIST of the greater omentum.