Late postoperative complications occurred after posterior fusion and posterolateral fusion as a result of biomechanical alterations. The stress change between the two fusion procedures has not been well reported. To differentiate the biomechanical alteration that occurs with posterior fusion and posterolateral fusion of the lumbar spine, the load sharing of the vertebrae, disc, facet joint, bone graft, and the range of motion were computed in a finite element model. Five finite element models, including the intact lumber spine, posterior fusion, posterior fusion with implant, posterolateral fusion, and posterolateral fusion with implant, were created for stress analysis. The finite element model estimated that the differences between these two fusion procedures were within 7% in stress of the adjacent disc, 3% in force of the facet joint above the fusion mass, and 5% in the range of motion. However, the stress of the pedicle in posterolateral fusion without an implant was at most two times greater than that in the intact lumbar spine under lateral bending. The stress of pars interarticularis in posterior fusion without an implant was also at most two times greater than that in the intact lumbar spine under lateral bending. After the implant was added, the discrepancy between the two fusion procedures decreased but still remained a relatively large difference. Therefore, the largest changes of posterior fusion and posterolateral fusion were in the pars interarticularis and pedicle, respectively.