This paper reports one case of primary fibrosarcoma affecting the mandible in a 41-year-old woman. Microscopically, the tumor was cellularized with an intense mitotic activity, and areas of necrosis and hemorrhage, and the cells showed immunoreactivity only for vimentin, establishing the diagnosis of primary intraosseous fibrosarcoma. The patient underwent tumor surgical resection, supraomohyoid neck dissection and mandible reconstruction with fibula flap and titanium bar. She also received post-surgical radiotherapy and is in follow up for 36 months without signs of recurrence or metastases.345579581