Background: The ratio of the diameter of the main pulmonary artery (mPA) to the diameter of the aorta (Ao) on chest computed tomography is associated with diverse clinical conditions. Herein, we determined the functional and prognostic implications of the mPA/Ao ratio in Korean chronic obstructive pulmonary disease (COPD) patients. Methods: The study population comprised 226 chronic obstructive pulmonary disease patients from the Korean Obstructive Lung Disease cohort who underwent chest computed tomography. We analyzed the relationships between the clinical characteristics, including pulmonary function, echocardiography findings, St. George's Respiratory Questionnaire, 6-minute walking (6MW) distance, and exacerbation with the mPA, Ao, and mPA/Ao ratio. Results: The mean age was 65.8 years, and 219 (96.9%) patients were male. The mean FEV1% predicted and FEV1/FVC ratio were 61.2% and 47.3%, respectively. The mean mPA and Ao were 23.7 and 36.4 mm, respectively, and the mPA/Ao ratio was 0.66. The mPA/Ao ratio correlated negatively with the 6MW distance (G = -0.133, P = 0.025) and positively with the right ventricular pressure (G = 0.323, P = 0.001). After adjustment for potential confounders, the mPA/Ao ratio was significantly associated with 6MW distance (β = -107.7, P = 0.017). Moreover, an mPA/Ao ratio >0.8 was a significant predictor of exacerbation at the 1-year (odds ratio 2.12, 95% confidence interval 1.27-3.52) and 3-year follow-ups (odds ratio 2.04, 95% confidence interval 1.42-2.90). Conclusions: The mPA/Ao ratio is an independent predictor of exercise capacity and an mPA/Ao ratio >0.8 is a significant risk factor of COPD exacerbation.