Introduction: Obesity has been linked to atrial fibrillation (AF) burden and severity, and epidemiological studies suggest that AF is more prevalent in whites than Asian or Afro-Caribbean populations. We aimed to investigate whether obesity mediates associations with AF in Europe and Asia using patient-level data comparisons of two cohort studies. Hypothesis: We therefore hypothesized that the difference in the distribution of BMI and WC can partially explain the ethnic differences in AF incidence Methods: Using Korean National Health Insurance Service’s Health Screening (NHIS-HealS) and U.K. Biobank data, we included 401,206 Korean and 477,926 British aged 40-70 years without previous AF who received check-ups. The incidence and risk of AF were evaluated regarding different body mass index (BMI) values. Results: The obese proportion(BMI ≥ 30.0 kg/m 2 , 2.8% vs. 24.3%, P < 0.001) was higher in the U.K. than the Korean. In the Korean and U.K. cohort, the age- and sex-adjusted incidence rates of AF were 3.52 and 3.95 per 1,000 person-years among overweight individuals (BMI, 25 to <30 kg/m 2 ), and 4.97 and 6.54 per 1,000 person-years among obese individuals, respectively. Compared to Koreans, the risk of AF was higher in the British population, with adjusted hazard ratios of 1.11 (95% confidence interval [CI], 1.04-1.19) and 1.16 (95% CI, 1.11-1.22) in overweight participants and 1.69 (95% CI, 1.57-1.83) and 1.45 (95% CI, 1.30-1.62) in obese participants, respectively ( P for interaction < 0.05). Conclusions: Obesity was associated with AF in both populations. British subjects had a greater incidence of AF related to the high proportion of obese individuals, but the risk of AF was also increased when separating participants into overweight and obese categories.