Fracture of the clavicle following radical neck dissection (RND) and/or radiotherapy is a rare complication. Several causes of fracture of the clavicle after treatment of head and neck cancer were postulated in previous reports. We present a case of fracture of the clavicle after treatment of squamous cell carcinoma of the tongue. An 81-year-old Japanese woman underwent RND, subtotal glossectomy, reconstruction using a pectoralis major myocutaneous flap (PMMCF), and postoperative radiotherapy (50.4 Gy). One month after the primary treatment, fracture of the clavicle occurred. It was thought that muscular dynamic factor and reduction of blood supply in the clavicle associated with RND and PMMCF were the causes of the fracture. We have to recognize the occurrence of this complication and try to reduce the factors related to the complication.