Biomechanical analyses under compressive load, flexion, and extension torque were performed, using a nonlinear three-dimensional finite element method, to evaluate stability in posterolateral fusion. Effects of facet fusion and disc denucleation on posterolateral fusion were also examined. Using an initially prepared L4-5 motion segment model, we prepared a denucleation model, posterolateral fusion models classified by presence or absence of denucleation and facet fusion, and an interbody fusion model. In the denucleation model, rigidity was less than in the normal model, and maximum rigidity was analyzed for the interbody fusion model. The effect of denucleation on posterolateral fusion was also analyzed. Taking into account the instability of the anterior elements, including the intervertebral disc, appears to be clinically important. In the posterolateral fusion model under compressive load, the axis of rotation moved principally toward the fusion mass, and axial displacement and flexion rotation were induced. Sagittal rotation angles under flexion and extension torque were 1.5 degrees -2.3 degrees at a maximum moment of 15 N-m, demonstrating elasticity of posterolateral fusion. When combined with facet fusion, posterolateral fusion yielded increase of load transfer across the lamina and decrease of rotation angle of about 10% under flexion-extension torque. Adjunctive clinical use of facet fusion should permit more solid posterolateral fusion.