Fibromatosis represents a group of fibrous tumors showing clinic and biologic features between benign fibrous lesions and fibrosarcoma. These locally aggressive tumors have high recurrence rates (20% to 70%). A four-year-old boy presented with mandibular fibromatosis occupying the mandible completely and extending to the submandibular gland and soft tissues. Complete hemimandibulectomy and submandibular gland excision were performed followed by reconstruction with a curved Kirschner wire. No signs of recurrence was observed during a follow-up period of 18 months. In addition, no limitations in the functions of the jaw, mastication, and swallowing were noted.