We report a new case of aggressive juvenile fibromatosis (FJA) in a 20 months old girl. It was initially treated by hemimandibulectomy. A secondary reconstruction was performed with a fibular flap at the age of 9 years. FJA is a locally aggressive lesion which does not metastasize. It occurs chiefly in girls during childhood. Radiographs are non-specific (extensive osteolytic lesion). The clinicopathologic diagnosis is discussed with the other mandibular fibrous tumors in children.