Background: pelvic incidence (PI) is a fundamental pelvic anatomic parameter that is specific and constant for each individual and determines pelvic orientation as well as the size of lumbar lordosis (LL). Pelvic incidence (PI) is a descriptor of pelvic morphology and not of pelvic orientation: therefore, its angular value is unaffected by changes in human posture. The pelvic tilt (PT) and the sacral slope (SS) are position-dependent variables and are very useful to characterize the spatial orientation of the pelvis. Pelvic incidence, sacral slope and pelvic tilt are particularly useful because it can be demonstrated that pelvic incidence (PI) is the arithmetic sum of the sacral slope (SS) + pelvic tilt (PT). Objective: this study aimed to determine the effect of Postero-Lateral Inter Body Fusion Surgery (PLIF) in 25 patients with Lumbo-Sacral instability by measuring these three pelvic parameters by plain x-ray before and after surgery. Patients and Methods: when compared to normal populations, pelvic incidence (PI) is significantly higher in spondylolisthesis and the difference in PI tends to increase in a direct linear fashion as severity of the spondylolisthesis increases. The cause effect relationship between pelvic morphology and spondylolisthesis remains to be clarified. Taking into consideration the pelvic parameters stimulated a renewed interest for the radiological evaluation and classification of spino-pelvic alignment in L5-S1 spondylolisthesis. Results: we found that Postero-Lateral Inter Body Fusion Surgery (PLIF) is sucssessful operation for treatment of lumbo sacral instability such as degenerative spondylolisthesis, isthmic spondylolisthesis and post laminectomy instability. In comparison with lateral view x-ray before and after the operation, the pelvic incidence increased and pelvic tilt decreased. Conclusion: PLIF is successfully used to treat degenerative and isthmic spondylolisthesis. PLIF is successful regardless of age, sex and concomitant morbidities such as diabetes mellitus, hypertension and ischemic heart disease.