Purpose: To compare the functional results of posterior cruciate-retaining (PCR) and posterior sub- stituting (PS) replacement in patients undergoing primary total knee arthroplasty (TKA) for osteoarthritis. Materials and Methods: A randomized controlled study was carried out to compare the clinical and radiological outcomes of PCR and PS primary P.F.C. Ⓡ TKA. One hundred and sixty eight patients (188 cases), who had a primary diagnosis of osteoarthritis, and an intact functioning posterior cruciate ligament, were enrolled in this study. The patients were randomized regardless of the level of posterior cruciate ligament preservation. One hundred and eighty eight cases had a minimum 6-year follow-up, which included 96 and 92 cases in the PCR and PS groups, respectively. Results: The Knee Society Score, the Hospital for Special Surgery (HSS) knee rating scores, average flexion contracture and radiologic alignment improved significantly, but there were no significant difference between the two groups. On the other hand, the mean postoperative flexion angles in the PCR and PS knees increased from 114.7 o to 121.2 o and 119.3 o to 131.3 o , respectively (p<0.05). Conclusion: There were no significant differences between PCR and PS TKA in the follow-up knee scores. However, the level of postoperative improvement in further flexion was better in the PS group.