CT is the most accurate and non-invasive method to estimate the diameters of ascending and descending thoracicaorta, and pulmonary artery in vivo. The study was undertaken to obtain standard values of the diameters andrelationships of those great arteries in Korean adults. 100 cases of chest CT in normal Korean Adults wereanalized and the results were as follows: 1. The average diameters of ascending and descending aorta were35.4±5.7mm, 29.4±3.8mm at the level of aortic root, respectively. 2. The average diameters of ascending anddescending aorta were 34.1±5.4mm, 24.4±3.5mm just below the aortic arch, respectively. 3. The ratio of ascendingaorta to descending aortic diameter were 1.56±0.23, 1.49±0.2 at the level of aortic root and just below theaortic arch, respectively. 4. The ratio of ascendidng aorta to descending aortic diameter decreased with age. 5.There were no statitstical difference of both ascending and descending aortic diameters between sex independent ofthe levels. 6. The average diameters of right and left pulmonary artery were 24.4±2.9, 15.9±2.5, respectively,and the diameter of the pulmonary artery was larger than the right side. 7. There were no statistical differenceof both right and left pulmonary artery diameters between sex. 8. The diameters of right and left pulmonary arteryincreased with age, but the diameter of right pulmonary artery was larger than left side on 7th decade.