Obesity is a known major contributing risk factor for knee osteoarthritis (OA). It is also believed that obese unicompartmental knee arthroplasty (UKA) patients tend to have poorer outcome and possible early failure. The purpose of this study is to investigate the early outcome of obese UKA patients in a single institution. Patients who underwent fixed bearing medial UKA in between year 2005 and 2010 were included in this study. They were divided into four groups based on Body Mass Index (BMI): 25 kg/m(2) (Control); 25-29.9 kg/m(2) (Overweight); 30-34.9 kg/m(2) (Obese); >35 kg/m(2) (Severely Obese). Functional outcome was assessed using Knee Society Score (KSS), Oxford Knee Score (OKS) and Short-form 36 (SF-36). One-way ANOVA with Bonferroni post-hoc test was used to compare the four groups for quantitative variables. There were 673 patients in this study, no significant difference between the four BMI groups for gender and side of operated knee (p >0.05). The functional outcome of all four groups at 2 years were comparable (all p >0.05). At a mean follow up of 5.4 (range 2.5, 8.5) years, 9 revision surgeries (1.3%) were identified. The mean duration from initial surgery to revision surgery was 49 months (Range 6, 90). Patients' pre-operative BMI did not influence the early outcome of UKA patients. However, patients with higher BMI had relatively lower functional score prior to the surgery and tended to be younger. This did not translate to early failure and the functional improvement was similar among all four groups.