We report the case of a 57-year-old man who developed osteoradionecrosis (ORN) at the occipitocervical (OC) junction after radiation therapy for nasopharyngeal carcinoma. During soft-tissue debridement using a nasopharyngeal endoscope, the anterior arch of the atlas (AAA) was spontaneously disrupted, which was later spat out. Radiographic examination revealed complete disruption of the AAA that caused OC instability. We performed posterior OC fixation. The patient experienced successful postoperative pain relief. AAA disruption secondary to ORN at OC junction can cause severe instability. Posterior OC fixation alone may be an effective procedure if the necrotic pharyngeal region is mild and endoscopically controllable.