Odontoid process fracture, nonunion or atlantoaxial instability are generally treated with posterior fusion using sublaminar wiring techinique. And occiput to C2 fusion is performed in cases with posterior arch defect of atlas. However, occiput to C2 fusion can not stabilize unstable C1-2 segment before accomplishment of fusion. Therefore, postoperative external support is necessary. A 48-year old male patient visited our hospital due to weakness and spasticity of four extremities. 6 months ago, he got C1-2 fusion in other university hospital due to odontoid process fracture. 6 months after surgery, sublaminar cable was pulled out and grafted bone was absorbed. The neurological deficits were worsened(spasticity of four extremities with severe myelopathy, bed ridden state). JOA score was 4. Diagnosis of the patient was C1-2 instability with cervical myelopathy due to odontoid process fracture nonunion and posterior arch defect of atlas. C1-2 transarticular screw fixation and occiput to C2 fusion were performed.