Backgrounds: Although prevalence of atrial fibrillation (AF) has been investigated in several epidemiologic studies, incidence data on AF are limited. Methods: We investigated that the prevalence and incidence rate of AF in the Korean Health and Genomic Study (KHGS), which is an ongoing population-based epidemiological study of Korean adults aged 40 to 69 years (yr). This cohort began in 2001 and initial examinations were performed from 2001 to 2002 on randomly selected participants. The participants were re-examined in two-year interval. Results: A total of 9826 subjects (mean age=52.2짹8.9 years, 4680 men and 5184 women) were enrolled in this study. The prevalence rate of AF were 0.39% in 1st examination (n=9826) and 0.52% in 2nd examination (n=8474). In gender and sex specific analysis at 1st moment, it was 0.62% in men and 0.17% in women, and 0.11% in aged 40-49 yr, 0.13% in 50-59 yr, and 1.04% in 60-69 yr. It was more prevalent in men than in women (OR 3.7, P=0.0006) and in aged 60-69 yr than in aged 40-49 yr (OR 8.1, P<0.0001). At 2nd moment, it was 0.82% in men and 0.25% in women, and 0.15% in aged 40-49 yr, 0.2% in 50-59 yr, and 1.36% in 60-69 yr. It was more prevalent in men than in women (OR 3.3, P=0.0006) and in aged 60-69 yr than in aged 40-49 yr (OR 6.9, P<0.0001). Two-year incidence of AF was 1.1/1000 person-yr (py, 1.5/1000 py in men vs. 0.6/1000 py in women, P=0.003). Conclusion: In this prospective cohort study of Korean population, the prevalence rate of AF was 0.39% and 0.52% at two moments, and the incidence of AF was 1.1/1000 py. These increased with age and were higher in men than in women.