Posterior Lumbar Interbody Fusion have standed the test of time as an effective method for managing lumbar instability. This study aimed to asses and evaluate the outcome of using a single cage in posterior lumbar interbody fusion as a surgical treatment modality for symptomatic lumbar instabilities. The study include 20 patients, 9 were males (45 %) but 11 were females (55%). Mean of age was 53 ±14, the mean BMI was 28.5. About one-quarter were smokers (25.0%). All patients had leg pain, and most of them (85%) had back pain. more than half of the studied patients showed L5/S1 affection. About one-third showed L4/5 affection. Only 10% and 5% showed L3/4 and L2/3 affection. VAS score of leg pain showed an overall significant difference. Oswestry Disability Index (ODI) score showed an overall significant difference. Post-operative complication showed that two patients showed cage posterior migration. One patient showed screw malposition, and one patient showed superficial infection. The Patients’ satisfaction reported that most patients (70.0%) reported excellent outcomes. Only 20% and 10% reported good or fair outcomes. Single cage in posterior lumbar interbody fusion for treatment of lumbar instability enables sufficient decompression and produce satisfactory clinical outcomes and radiological outcomes.