Vertical sacral fracture is one of the most difficult fractures to treat. Posterior fixation using spinal dual rods is a novel method for treating this fracture, but its biomechanical strength has not yet been reported. The aim of this study was to evaluate the biomechanical strength produced by posterior fixation using spinal instrumentation. Sacral fractures were created in eight pelvic bone models and classifi ed into a posterior plate fixation group [P group, n＝4] and a spinal instrumentation group [R group, n＝4]. The biomechanical strength was tested by pushing down on the S1 vertebra from the top. The mean maximum loads were 1,057.4 N and 1,489.4 N in the P and R groups, respectively (p＝ 0.014). The loads applied to the construct at displacements of 5mm and 7.5mm from the start of the universal testing machine loading were also significantly higher in the R group. The mean stiffness levels in the P and R groups were 88.3N/mm and 119.6N/mm, respectively (p＝0.014). Posterior fixation using spinal instrumentation is biomechanically stronger than conventional posterior plate fixation. This procedure may be the optimal method for treating unstable sacral fracture fixation.