This study was aimed at predicting moments with respect to diameters of wires for evaluating stability of the upper cervical spine fixed with wires based on the finite element analysis. In case of the severe atlanto-axial instability, several surgical methods have been tried and the posterior fixation using wires has been widely used because of the sufficient stability and high rate of bony adhesion. The diameters of wires applied in this study were 0.7 mm, 0.9 mm and 1.25 mm. 1.5 Nm is the moment for the normal physiological range of motion of the upper cervical spine in cadaver models. However, if this moment is applied to the occiput, an excessive load occurs at the occipito-atlantal joint and clinical problems can break out realistically. Thus, it is necessary to predict moments for evaluating stability with respect to diameters of wires. The results showed that 0.7 mm wire allowed the biggest moment while 1.25 mm wire allowed the smallest moment. In addition, the upper cervical spine fused with wires was stabilized effectively as the load increased.