Objective: To evaluate the relationship between postoperative knee function and the sagittal position of tibial component in unicompartmental knee arthroplasty (UKA). Methods: We retrospectively enrolled the patients who underwent UKA from January 2016 to May 2020. They were assigned into 2 groups according to postoperative posterior tibial slope (PTS): the normal PTS group (PTS≥3° and PTS<8°) and the abnormal PTS group (PTS<3° or ≥8°). The patients were followed up for at least 12 months. The postoperative Knee Society Clinical Score (KSS-C), Knee Society Functional Score (KSS-F) and knee range of motion (ROM) were compared between the two groups. Results: A total of 72 patients (82 knees) were included with 51 patients (58 knees) in PTS normal group and 21 patients (24 knees) in PTS abnormal group. All the patients were followed up with median of 23.6 months. There was no significant difference in the general data [gender, age, body mass index (BMI)], pre-operative knee range of motion, preoperative KSS-C score and KSS-F score (P > 0.01). The KSS-C score, KSS-F score, and knee range of motion significantly improved after surgery (P<0.01) for all the patients. The postoperative KSS-C score in normal PTS group (88.76±2.79) was significantly higher than the KSS-C score in abnormal PTS group (84.42±3.35, P<0.01), but no significant difference between the 2 groups was observed in postoperative KSS-F score and knee range of motion (P > 0.01). In addition, there was no correlation between the change of PTS and postoperative KSS-C score (r=-0.034, 95%CI: -0.247 to 0.186, P = 0.759), KSS-F score (r = -0.014, 95%CI: -0.238 to 0.198, P = 0.901) and knee range of motion (r= 0.045, 95%CI: -0.214 to 0.302, P = 0.686). Conclusion: The posterior tibial slope between 3° and <8° can be recommended to improve knee joint function in mobile UKA, and excessive or insufficient PTS should be avoided.