Osteosarcomas of the cranial bones need a large surgical radical resection. The best option to reconstruct mandible defect after resection is the free fibula flap. In our patient an acute ischaemic leg occurred just after the free fibula flap harvest for mandible reconstruction. The abnormal distribution of the calf arteries leads to catastrophic consequences. The peroneal artery could be the main dominant artery of the leg in a small number of patients. We reported an extremely rare case of "peronea magna", described in less than 0.2% of the global population. A careful pre-operative workup of the calf vessels is required in all the patients who need free fibula flap harvest.