Background: Fusion of the spine is a type of treatment option when the movement of the spine is the source of the pain-the theory being that if the painful vertebrae do not move, they should not hurt.Posterior interbody fusion has many advantages compared to other methods of fusion. Aims and Objective:To study the functional outcome of lumbar diseases treated with single-level instrumented posterior lumbar interbody fusion. Material and Method: A hospital-based prospective study was conducted in the Department of Orthopedics at a tertiary care center from November 2019 to May 2021. The patients were informed about the study in all respects and informed written consent was obtained. Follow up period was for 6 months. Results: A total of 30 patients were included in our study. The radiological union rate was found to be 73.3 percent. The average time taken for the procedure was 3.5 hours. The average blood loss was 237 milliliters. The improvement in the postoperative VAS score at the six-month mark was drastic and significant, as proven by a "p-value" of < 0.0001. Improvement in quality of life, an assessment, based on the Wilcoxon signed-rank test comparing Oswestry Disability score (ODS) and Oswestry Disability Index (ODI) preoperatively and postoperatively, was statistically significant, showing a reduction in Oswestry Disability index and score postoperatively, indicating significant improvement in the quality of life. Conclusion: PLIF is used to treat a degenerative disc with narrowing disc space, spinal canal stenosis, or a case of spondylolisthesis that hasn't responded to conservative treatment. In light of the results and minimal complication rate, we would recommend the PLIF technique combined with bone grafting as an appropriate technique for spondylolisthesis and degenerative disc disease.