Introduction: Pulmonary hypertension has been associated with a significant increase in morbidity and mortality. The pulmonary artery diameter measurement from a CT scan is routinely used to predict possible evidence of pulmonary hypertension. However, no data available as a reference for the normal range, especially in the Thai Population. The primary purpose of this study was to determine the normal diameter of the main pulmonary artery (MPA) and ascending aorta (AAo) and the ratio between MPA and AAo diameter in the Thai population. Methods: Patients who met the inclusion criteria and had a chest CT performed in Songklanagarind hospital between 1 January 2014 and 14 July 2014 were enrolled. Measurement of the MPA diameter at the level of its bifurcation and the AAo from an axial-view contrast-enhanced CT chest (venous phase) image was done. The ratio between the diameter of MPA and AAo was then calculated. Results: 2395 patients were included in this study (male = 1271 patients, female = 1124 patients, aged between 20-96 years; mean = 57 years old). The mean diameter of the MPA was 25.21 mm, and the mean diameter of the AAo was 30.78 mm. The mean MPA/ AAo ratio was 0.83. The sex-specific mean diameters of the MPA were 25.4 mm in men and 25.0 mm in women. The mean diameters of the AAo in men and women were 31 mm and 30 mm, respectively.Conclusion: The mean MPA diameter obtained from chest CT scans in this study was about 25.4 mm in men and 25.0 mm in women. These values are easily obtained from chest CT scans and are highly reproducible. Therefore, it may be used as a reference value for a healthy Thai population.