Abstract: Objective No reports on experience with treating nontraumatic chronic occipitoatlantal dislocation(OAD) in adults can be found. We reported two cases treated successfully to improve the diagnosis and surgical treatment of nontranmatic chronic OAD. Method The two patients(58 yrs, female & 18 yrs, male) presented with progressive weakness in both sides of upper and lower limbs for 10 years and half a year respectively, without history of trauma. CT and MRI confirmed the diagnosis of OAD with syringomyelia. Surgical procedures included lower clivus removal via transoral approach, cranial traction, and posterior occipitocervical fixation and fusion. Results Clinically, neurological deficits were remarkably improved. Postoperative imaging studies showed good realignment of occipito-atlanto-axial complex with comfortable decompression of brainstem. The syringomyelia was relieved remarkably. Conclusions Anterior tranoral decompression by removing the lower part of the clivus combined with cranial retraction and posterior fixation can be adopted in the treatment of nontraumatic chronic OAD.