Biomechanical analyses under compression only, and for a combination of flexion, extension, rotation, and lateral bending were performed to evaluate the stress of the interarticular portion of the lumbar vertebra using a nonlinear three-dimensional finite element method. A detailed three-dimensional L4-L5 motion segment model was developed that took into consideration the material nonlinearities of ligaments and annular fibers and the contact nonlinearities of facet joints. For a more accurate examination, the separation of cortical bone and cancellous bone for both posterior and anterior elements were also considered. The stress in the pars interarticularis was weakest under compression alone, but stronger under compression with lateral bending loading, with flexion, with rotation, and with extension. Under each loading condition, the region of the stress concentration was consistent with the separated region of the spondylolysis observed in clinical situations. Since the stress in the pars interarticularis was high under extension and rotation in particular, those loadings were suggested to be relatively high risk factors leading to spondylolysis.