Abstract:[Objective] To explore the impact of obesity on the clinical outcome of primary unicompartmental knee arthroplasty (UKA) for medial knee osteoarthritis. [Methods] A retrospective study was done on 196 patients who underwent primary unilateral UKA for medial knee osteoarthritis from July 2016 to June 2018. According to preoperative body mass index (BMI), the patients were divided into three groups, including normal group (n=41) (BMI 18.50~24.99 kg/m2 ) , overweight group (n=113) (BMI 25.00~29.99 kg/m2 ) , and obese group (n= 42) (BMI ≥30.00 kg/m2 ) . The documents regarding to perioperative period, follow- up and radiographs were compared among the three groups. [Results] All patients in three groups had UKA performed successfully, without serious complications. Perioperative adverse events were of 2/41 (4.88%) in the normal group, 6/113 (5.31%) in the overweight group, and 2/42 (4.76%) in the obesity group, which did not lead serious consequences after corresponding treatment, and were not statistically significantly different among the three groups (P>0.05) . All the patients in the three groups got well incision healing, without postoperative wound infection in anyone of them. During the follow up last- ed for more than two years, with an average of (35.12±6.53) months, no prosthetic failure, nor revision surgery happened in anyone of the pa- tients in three groups. Compared with that preoperatively the WOMAC score in the three groups was significantly improved at the latest fol- low-up (P<0.05) , whereas which proved not significantly different among the three groups at any corresponding time points (P>0.05) . At the last follow-up, all patients in the three groups were satisfied with the operation without significant difference in subjective satisfaction score among the three groups (P>0.05) . In terms of imaging, the hip- knee-ankle angle (HKAA) at the last follow- up was of (175.86± 2.50) ° in the normal group, (177.30±2.66)° in the overweight group, and (176.23±2.52)° in the obese group, with a significant overall differ- ence among the three groups (P=0.010) . However, there was no significant difference in HKAA between the obese group and the normal group (P=0.563) . [Conclusion] Obesity does not impact clinical outcome of UKA significantly for medial knee osteoarthritis, in other word,the obese patients can still get good clinical outcome after UKA.