Though leiomyosarcoma usually occurs in the gastrointestinal tract and uterus, it rarely occurs in the wall of large veins and arteries. We present a case of primary leiomyosarcoma arising in the great saphenous vein of the left inner thigh and spreading for some extent along the vein in a 54 year old female. Diagnosis was confirmed by desmin stain and electron microscopy. Postoperative course was fine. This is the first report of this in Korean literature.