In this report. a case of leiomyosarcoma in a 3-years·oId, male. crossbred dog was described. Macroscopically. the tumor mass was measured as 8x3x3 cm in dimensions and was located in the back region. Tumor was graYlsh·white colored. solitary, subcutan, and had a lobular structure. Microscopically. tumor composed of the neoplastic calls With abundant cytoplasma, blunt and central nucleus. Immunohistochemically. some tumor markers, such as smooth muscle actin, desmin. vimentin and 5-100 protein were investigated, from these Vlmantin was found to be positive.