||Background The pedicle screw fixation system is the gold standard treated method in clinical for the unstable disease. Generally, pedicle screw can be classified into two categories included monoaxial and polyaxial constructions. Although the polyaxial screws were common used owing to it can provide more degree of freedom on the screw-to-rod connection to facilitate rod seating. However, the complexity of structural design may reduce their mechanical strength. The size of slicing spheres of polyaxial screw head was found as the important issue to influence the mechanical strength of the pedicle screw system. Therefore, this study investigated the effects of different size of slicing spheres of polyaxial screw head on the vertebrectomy model under compression test. Material and Method The different size (60, 75 and 90% diameter) of slicing spheres of polyaxial screw head were assembled for destructive mechanical testing. All pedicle screw systems assemblies were tested in compression test according to FDA regulation testing method of ASTM F1717. The stiffness and ultimate load were determined for different slicing spheres of screws. Result and discussion The results of testing found that stiffness were 10.91 ± 2.03, 19.34 ± 1.21, and 23.39 ± 0.85 N/mm and ultimate loads were 185.43 ± 14.57, 264.44 ± 21.68, and 364.54 ± 10.43 N for 60, 75, and 90% diameter, respectively. Significant differences (p < 0.05) were found among these three groups. Conclusion The mechanical strength presented by stiffness and ultimate load increased as the size of slicing spheres of polyaxial screw head increased.