Background: Osteoarthritis (OA) is the most common joint disease and most frequent cause of chronic muscloskeletal pain. When conservative treatments fail, different types of surgical interventions for medial compartmental O.A. are valid including; high tibial osteotomy (HTO), unicompartmental knee arthroplasty (UKA) or total knee arthroplasty (TKA). The aim of this work was to assess the ability to achieve the anatomical posterior tibial slope in manual medial UKA and to asses effect of posterior tibial slope (PTS) on postoperative knee range of motion. Methods: This prospective randomized study was carried out on 40 patients with medial knee compartment osteoarthritis prospectively who were candidate for medial UKA at Benha University Hospitals. Patients were randomly divided into two equal groups; Group 1: patients preoperative anatomical tibial slope angle were tried to be achieved manually and group 2 where standard 7° angle PTS suggested by the manufacturer were used. All patients were subjected to general and musculoskeletal examination, laboratory investigations and radiological evaluation. Results: There was a significant positive correlation between postoperative PTS and postoperative range of motion (ROM) flexion (r=0.849, P