Os odontoideum is a rare anomaly of the second cervical vertebra, which can result in the compression and injury of cervical spinal cord. This deformity is surgically challenging. The authors presented a case of a 50-year-old man with a 2-year history of numbness and weakness in four limbs. The x-ray suggested the os odontoideum. MRI demonstrated a dramatic compression of the cervical spinal cord and an abnormally high signal intensity area in this region. The patient underwent a midline posterior approach for the treatment of this lesion. Postoperatively, the reduction results were satisfactory and the compression was relieved.