[Objective] To investigate the effect of lumbar sacralization on lumbar isthmic spondylolisthesis. [Methods] From January 2007 to July 2018, 125 patients who underwent surgical intervention for L4 spondylolysis were analyzed retrospectively. 59 patients were accompanied with lumbar sacralization, and they were further divided into two groups according to the Castellvi classification. The degree of vertebral slip and degeneration was compared between the sacralization group and the no-sacralization group, as well as between the Castellvi type II group and type III group. [Results] There was no significant difference in sex and age between the sacralization group and the no-sacralization group. The degree of vertebral slip was (33.32±14.42) % in the sacralization group and (23.76±10.25) % in the no-sacralization group. There was significant difference in the degree of vertebral slip between groups (t=4.229, P<0.001). The sacralization group showed a more severe disc degeneration than the no-sacralization group (Z=3.095, P=0.002). The Castellvi type II group had 23 cases, and the type III group had 36 cases. There was no significant difference in sex and age between the groups. The Castellvi type II group showed a less vertebral slip compared with the type III group (t=-2.154, P=0.035). There was no significant difference in the degree of degeneration between groups. [Conclusion] The degree of vertebral slip and degeneration was aggravated by lumbar sacralization, and the greater the osseous bridging, the more severe slippage the L4 vertebra seemed to have.