The purpose of this study was to analyze the effect of lumbar spine fusion on the superior adjacent intervertebral disk in the context of disk degeneration, using a nonlinear three-dimensional finite element method. Detailed L3-L5 motion segment models of normal and degenerated intervertebral disks were developed. In fusion models, L4-L5 was fixed by either posterolateral fusion or posterior lumbar interbody fusion (PLIF). Various loading conditions such as compression loading, compression loading plus flexion moment loading, or compression loading plus extension moment loading were applied to study the corresponding stress. Tresca stress on the posterolateral part of intervertebral annulus fiber and von Mises stress on the vertebral endplate (the superior and inferior sides of L3 and L4) were reduced in all degenerated disk models compared with the normal disk models. The PLIF model showed an increase in the percentage change of stress on the vertebral endplate and on the intervertebral annulus fibrosus when flexion and extension moment loadings were applied. This finding suggests that surgeons should consider the risk of exacerbating degeneration of intervertebral disks by undertaking lumbar spine fusion, when degeneration is found in intervertebral disks adjacent to vertebrae requiring fusion.