Computed tomography is the most accurate and non-invasive method to estimate the dia-meters of ascending and descending thoracic aorta and pulmonary artery in vivo, The study was undertaken to obtain standard values of the diameters and relationships of those great arteries in Korean adults. 100 cases of chest CT in normal Korean Adults were analized and the results were as follows: 1. The average diameters of ascenidng and descending aorta were 35.4$pm$5.7mm 29.4$pm$3.8mm at the level of aortic root respectively. 2. The average diameters of ascending and descending aorta were 34.1$pm$ 5.4mm24.4$pm$3.5mm just below the aortic arch respectively. 3. The ratio of ascending aorta to descending aortic diameter were 1.56$pm$0.23 1.49$pm$0.2 at the level of aortic root and just below the aortic arch respectively. 4. The ratio of ascending aorta to descending aortic diameter decreased with age. 5. There were no statistical difference of both ascending and descending aortic diameters between sex independent of the levels. 6. The average diameters of right and left pulmonary artery were 14.4$pm$2.9, 15.9$pm$2.5 respec-tively and the diameter of the pulmonary artery was larger than the right side. 7. There were no statistical difference of both right and left pulmonary artery diameters between sex. 8. The diameters of right and left pulmonary artery increased with age but the diameter of right pulmonary artery was larger than left side on 7th decade.