Background: The spondylolisthesis with or without spondylolysis can occur in 5% to 8% of adult population. It is most commonly believed that spondylolysis does not exist from the birth. Its incidence is 4.4% to 5% at age of 6 years, increasing it to 6% at age of 18 years, which is quite similar to one observed in adulthood.To analyze functional outcome of the posterior stabilization by the pedicle screws with the decompression and the interbody fusion by the PLIF (Posterior Lumbar Interbody Fusion) technique in the adult using the Modified Oswestry Disability Index. In this study we aim to analyze correlation between degrees of the fusion with the functional outcomes. Methods: This is a prospective study of 25 patients conducted in the Department of Orthopedics, D.Y. Patil Hospital, Kolhapur from August 2016 to June 2018. All the patients were evaluated with baseline anteroposterior, lateral and oblique radiographs of lumbosacral spine. MRI evaluation of all the patients was done to evaluate joint pathology, sacralisation/ lumbarisation, the condition of disc, neural canal and foramina stenosis and nerve root involvement. Pre and post -operative Modified Oswestery low back pain scores and Visual Analogue Score were assessed. Results: Out of 25 patients of degenerative and lytic listhesis of lumbosacral spine, after surgery, radiculopathy was relieved in 23 (92%) cases. Preoperative mean Modified Oswestery low back pain score of lumbosacral spine degenerative and lytic listhesis patients was found to be 53.96±11.5 and postoperative after 1.5 months, it was 30.48±13.07. Conclusions: Score was reduced significantly after surgery (Posterior Instrumentation, Bone grafting and Decompression) showing major improvements in symptoms among patients.