Fibrosarcoma is a malignant mesenchymal neoplasm of fibroblasts that rarely affects the oral cavity and can cause local recurrences or metastasis. In this article we describe a case of primary fibrosarcoma in the mandible in a 16-year-old girl. Microscopically the tumor showed an intense proliferation of spindle-shaped cells, varying little in size and shape and arranged in parallel bands, partly crossing each other, the mitotic activity was increased and there was nuclear pleomorphism. Immunohistochemically the cells only showed immunoreactivity for vimentin and negativity for S-100 protein, CD 68, cytokeratin cocktail, HMB-45, CD34, pan actin HHF 35, desmin, smooth muscle actin and epithelial membrane antigen (EMA). Based on clinical, radiological, histological and immunohistochemical findings the final diagnosis was high-grade intra-osseous fibrosarcoma. The treatment choice was radical surgery with mandibular reconstruction. After one year and nine months of the treatment the patient displayed multiple metastases. Radiation therapy and chemotherapy were used as adjuvant treatment. Unfortunately, the girl died two years after initial diagnosis.