For more than 80% of body weight is transmitted through the anterior and middle column, the interbody gap produced by operative reduction of spondylolisthesis is potential risk factor for redisplacement, implant failure, pseudoarthrosis and kyphosis. In biomechanical aspect, augmentation of the anterior column support by interbody fusion is desirable to prevent above problems in surgical treatment of spondylolisthesis. Recently, circumferential fusion is recommended in high-grade spondylolisthesis (grade III, IV, or V ), but there is some controversy about additional interbody fusion in posterior instrumentation for low-grade spondylolisthesis (grade I or II ). So, the purpose of this retrospective study was to compare the segmental sagittal angle in 35 patients of low-grade spondylolisthesis who was performed posterior instrumentation with or without posterial lumbar interbody fusion (PLIF) and to find out the risk factors of the loss of segmental sagittal angle in cases of posterior instrumentation without posterial lumbar interbody fusion (PLIF).