Fibrosarcoma is a malignant mesenchymal neoplasm of fibroblasts that rarely affects the oral cavity and can cause local recurrences or metastasis. In this paper, a case of fibrosarcoma in the posterior area of mandible in a 44-year-old woman is described. Clinical examination revealed a growth on right mandibular third molar region extending on the buccal and the lingual side. There was history of extraction of posterior teeth. Radiologically, there was a diffuse bone loss. Microscopically, the tumor showed an intense proliferation of fibroblasts of variable size and shape. These cells were arranged in parallel bands and partly crossing each other. The cells exhibited increased mitotic activity and nuclear pleomorphism. Immunohistochemically the cells showed immunoreactivity only for vimentin while they exhibited negativity towards S-100 protein, cytokeratin cocktail, HMB-45, desmin, smooth muscle actin, and epithelial membrane antigen (EMA). Based on findings the final diagnosis of fibrosarcoma was made.