Background & Importance: Atlas assimilation is the congenital Atlas fusion with the base of the occipital bone. Case Presentation: A patient was presented with neck pain, dysaesthesia and tingling of hands and feet and difficulty in walking for 4 years. On examinations, the patient had a short neck with rigidity in neck muscles and there were exaggerated jerks with bilateral Babinski sign. Cervical spinal x-ray indicated basilar invagination with the absence of a C1 arch and CT scan of the cervical spine showed assimilation of C1 with basilar invagination. Occipitocervical stabilization has been commonly proposed for the operational fusion. The iliac crest interposing bone graft was performed for fusion and stabilization of the occipitocervical region. The patient was treated surgically and soon after operation became painless and was able to move easily. Conclusion: In C1 assimilation with basilar invagination, bicortical occipital screws and C2 pedicle screws with interposition bone graft can be the way to optimize the shortest segmental stabilization and fusion procedure for occipitocervical fusion.