[Objective] To investigate the effect of obesity on clinical outcome of unicompartmental knee arthroplasty (UKA) for knee osteoarthritis. [Methods] A retrospective study was conducted on 196 patients undergoing medial UKA from July 2016 to June 2018. The follow-up time were 28 to 50 months, with an average of (35.12±6.53) months. The patients were divided into 3 groups based on preoperative body mass index (BMI): 41patients in normal group (BMI 18.50-24.99kg/m2), 113 patients in overweight group (BMI 25.00-29.99kg/m2), and 42 patients in obese group (BMI≥30.00kg/m2). No significant difference were seen in gender, age, and surgical sides among these 3 groups (P＞0.05). Data including postoperative complications, length of stay, satisfaction score, WOMAC score, and hip-knee-ankle angle (HKAA) were recorded. [Results] There was no significant difference in rate of postoperative complications and length of stay among 3 groups (P＞0.05). The satisfaction score was high in all 3 groups with no significant difference (P＞0.05). There was no significant difference in preoperative WOMAC score among 3 groups (P＞0.05). Postoperative WOMAC score was (9.02±7.69), (10.31±10.50), and (11.36±12.27) in normal, overweight, and obese group, respectively, with no significant difference (P＞0.05). Postoperative WOMAC score was significantly improved compared to preoperative WOMAC score in each group (P＜0.05). There was no revision surgery in all 3 groups. Postoperative HKAA was (175.86±2.50)°, (177.30±2.66)°, and (176.23±2.52)° in normal, overweight, and obese group, respectively, with significant difference (P＜0.05). [Conclusion] Obesity does not influence clinical outcome of medial UKA significantly. Obese patients can get good clinical outcome after UKA.