Abstract:[Objective] To compare the clinical outcomes of atlantoaxial fusion versus occipitocervical fusion for os odontoideum com- plicated with atlantoaxial dislocation. [Methods] A retrospective study was done on 29 patients who underwent posterior fusion for os odon- toideum complicated with atlantoaxial dislocation from January 2010 to December 2015. According to the concrete pathology and the re- sults of preoperative doctor-patient communication, 22 patients received atlantoaxial fusion (the AA group) , while the remaining 7 patients had occipitocervical fusion performed (the OC group) . The perioperative, follow- up and imaging data were compared between the two groups. [Results] All patients in both groups were successfully operated on without serious complications, and the atlantoaxial fusion group was significantly superior to the occipitocervical group in term of incision length (P<0.05) . All patients in both groups were followed up for more than 6 years, and the AA group resumed full weight-bearing activity significantly earlier than the OC group (P<0.05) . Cervical flex- ion-extension ROM and left-right rotation ROM were significantly reduced in both groups after surgery compared with those preoperative- ly (P<0.05) . The AA group proved significantly superior to the OC group in term of cervical flexion-extension ROM postoperatively (P< 0.05) . The NDI scores decreased significantly (P<0.05) , while JOA score increased significantly in both groups over time (P<0.05) , howev- er, there were no statistically significant differences in NDI and JOA scores between the two groups at any corresponding time points (P> 0.05) . Radiographically, the C0~2 angle increased significantly (P<0.05) , whereas the C2~7 angle and anterior atlantodental interval (ADI) de- creased significantly postoperatively compared with those preoperatively in both groups (P<0.05) . The AA group got less ADI than the OC group postoperatively, which proved statistically significant at 5 years after surgery (P<0.05) . [Conclusion] Both atlantoaxial fusion and oc- cipitocervical fusion are safe and effective for this disorder. By comparison, the atlantoaxial fusion takes advantages of reducing surgical trauma, improving atlantoaxial reduction, retaining more cervical motion, and getting better functional recovery.