Objective To investigate whether the functional outcomes were affected by the change in posterior tibial slope (PTS) after using a predetermined PTS for primary cruciate-retaining total knee arthroplasty (CR-TKA). Methods Prospective cohort study of 152 patients who underwent primary CR-TKA with a standardized PTS of 5º regardless of the native PTS. Patients were classified postoperatively in two ways. Firstly, according to the PTS change from preoperative to postoperative (increased or decreased PTS group). Secondly, according to the PTS difference between preoperative and postoperative ≤ 4º (group A) and > 4º (group B). The functional outcomes were assessed with the Knee Society Scores (KSS), McMaster Universities Osteoarthritis Index questionnaire (WOMAC), and range of motion (ROM). Preoperative and postoperative PTS were measured on lateral knee radiographs. Results The minimum follow-up was 5 year. There were no significant differences at the final follow-up in functional outcomes between increased (88 patients) and decreased (64 patients) PTS groups. Likewise, there were no significant differences in functional outcomes between group A (79 patients) and group B (73 patients). In multivariate analysis, the PTS change was not significant predictor for improvement in functional outcome (OR 1.08; 95% CI 0.70–1.40; p  = 0.061). Conclusion The PTS change between preoperative and postoperative has no influence on the functional outcomes using a CR-TKA. A standardized PTS regardless of the native is a reliable procedure for primary CR-TKA.