The rationale behind lumbar fusion surgery is to eliminate pathologic segmental motion and its accompanying symptoms, especially low back pain (LBP).Posterolateral fusion (PLF) using pedicle screw (PS) fixation has been one of the most popular procedures among the posterior lumbar reconstruction techniques. Lumbar interbody fusion is a recognized surgical technique in the treatment of chronic LBP associated with segmental instability. Purpose: The purpose of this study was to compare the clinical and radiological outcomes of stabilizing the spine with circumferential fusion using transforaminal lumbar interbody fusion (TLIF) versus posterolateral fusion (PLF) for lumbar segmental instability. Study Design: A prospective, nonrandomized clinical controlled trial. Patients and Methods: Forty patients with segmental lumbar instability were divided into two groups; the posterior interbody fusion group (PIF) in the form of TLIF and the PLF group; twenty patients each. Top loaded pedicle screw construct was used in both groups. The mean age of the patients was 48.35 years in TLIF group and 45.3 years in PLF group. Sex distribution was 6 males and 14 females in TLIF group and 7 males and 13 females in PLF group. Mechanical LBP was the chief complaint in all patients. Sciatica was a complaint of 12 patients (60%) of TLIF group and 13(65%) patients of PLF group. Patients were evaluated pre and postoperatively by visual analogue scale (VAS), Oswestry disability index (ODI) and radiographs. Results: The average operative time was 214.5 minutes in TLIF group and 192.5 minutes in PLF group. The mean estimated blood loss was 278 ml in TLIF group and 259 ml in PLF group. The average length of hospital stay was 3.85 days in TLIF group and 3.8 days in PLF group. Patients progressively improved regarding VAS and ODI in both groups with no statistical significant difference except for VAS for back pain where TLIF group gave better results. However, TLIF group gave better results in patients with secondary instability (e.g. postlaminectomy instability) than PLF group. Solid fusion occurred in 17 patients (85%) of TLIF group and 16 patients (80%) of PLF group with no statistical difference. Conclusion: Both transforaminal lumbar interbody fusion and posterolateral fusion are good options for the treatment of segmental lumbar instability. However, interbody fusion gives superior results in patients with secondary type instability.