Objective:To study the clinical problems about posterior atlanto-axial intemafixation and fusion for atlanto-axial instability or dislocation.Methods:Surgical treatments of 45 cases with atlanto-axial instability or dislocation were reviewed.There were 14 cases of odentoid malformation, 20 cases of odentoid fracture or rupture of transverse ligament, 11 cases of subluxation and rotation.All cases were treated using Gallies technique and fixed with plaster paris brackets.The followed-up period is 1 to 7 years with an average of 2 year and 3 months.Results According to Sumis criteria, excellent 19 cases(42.2%), good 13 cases(28.9%), fair 8 cases(17.8%), poor 5 cases(11.1%).There were 3 cases with bone graft postponed fusion cured by enhance externafixation.1 cases with nonunion were treated with revision surgery.No cases happened complication of cord injury.Conclusion:Gallies fusion technique is an effective method to manage the atlanto-axial instability or dislocation.Skull distraction before operation and reliable externalfixation post operative are important assistant measures.Key points for successful operation are careful wiring or cable traversing, decortica of posterior arc of C 1, and maintaining the physiological height between C 1 and C2 posterior arc.Indications and objectives should be conformed before revision surgery for failure cases