The main pulmonary artery diameter (mPA) and ratio of mPA to the aorta diameter (mPA/Ao) are routinely imaged and described in every patient undergoing chest computed tomography (CT). They give us information on the possibility and disease status of diverse underlying illness. However, there has been no large population study to determine the normal reference range of these parameters in Korea. We determined the distribution of normal reference values for mPA and mPA/Ao by CT in Korean population. A noncontrast, ECG-gated, coronary calcium scoring CT imaging of 2550 subjects who visited health screening center in Severance hospital, Seoul, Korea were used for measurement. Two experienced reviewers measured by the mPA and Ao at the level of bifurcation of main pulmonary artery and calculated the mPA/Ao. There were 1526 (59.8%) men and 1024 (40.2%) women with mean age of 53.1±9.4 years. The mean mPA was 26.6±3.0 mm and mean PA/Ao was 0.85±0.13. mPA is higher in men (27.0±3.4 mm vs. 26.0±3.4 mm, P＜0.0001), obesity (29.3±4.0 mm vs. 26.5±3.3 mm, P ＜0.0001), hypertension (27.6±3.6 mm vs. 26.3±3.3 mm, P＜0.0001), and diabetes mellitus group (28.2±3.5 mm vs. 26.5±3.4 mm, P＜0.0001). mPA/Ao is higher in women (0.86±0.13 vs. 0.84±0.13, P＜0.0001), obesity (0.89±0.13 vs. 0.84±0.13, P ＜0.0001), not hypertensive group (0.86±0.13 vs. 0.80±0.12, P＜0.0001), and not dyslipidemic group (0.85±0.13 vs. 0.79±0.13, P＜0.0001) with statistical significance. The main pulmonary artery diameter (mPA) and ratio of mPA to the aorta diameter (mPA/Ao) are routinely imaged and described in every patient undergoing chest computed tomography (CT). They give us information on the possibility and disease status of diverse underlying illness. However, there has been no large population study to determine the normal reference range of these parameters in Korea. We determined the distribution of normal reference values for mPA and mPA/Ao by CT in Korean population. A noncontrast, ECG-gated, coronary calcium scoring CT imaging of 2550 subjects who visited health screening center in Severance hospital, Seoul, Korea were used for measurement. Two experienced reviewers measured by the mPA and Ao at the level of bifurcation of main pulmonary artery and calculated the mPA/Ao. There were 1526 (59.8%) men and 1024 (40.2%) women with mean age of 53.1±9.4 years. The mean mPA was 26.6±3.0 mm and mean PA/Ao was 0.85±0.13. mPA is higher in men (27.0±3.4 mm vs. 26.0±3.4 mm, P＜0.0001), obesity (29.3±4.0 mm vs. 26.5±3.3 mm, P ＜0.0001), hypertension (27.6±3.6 mm vs. 26.3±3.3 mm, P＜0.0001), and diabetes mellitus group (28.2±3.5 mm vs. 26.5±3.4 mm, P＜0.0001). mPA/Ao is higher in women (0.86±0.13 vs. 0.84±0.13, P＜0.0001), obesity (0.89±0.13 vs. 0.84±0.13, P ＜0.0001), not hypertensive group (0.86±0.13 vs. 0.80±0.12, P＜0.0001), and not dyslipidemic group (0.85±0.13 vs. 0.79±0.13, P＜0.0001) with statistical significance.