Advanced cancer of the oral cavity has been treated with wide excision in conjunction with mandibulectomy and neck dissection. This has resulted in significant mandibulofacial defects with functional and cosmetic significance. Therefore, proper mandibular reconstruction is very important for physiologic and esthetic restoration. The risk factors of free flap reconstruction have been reported including obesity, age, smoking, previous irradiation, and systemic vascular disease. We recently experienced a case of donor site necrosis after fibular osteocutaneous free flap in oral cavity cancer.