Purpose: Aortic root size is important parameter in vascular diseases and can be easily assessed by transthoracic echocardiography. However, measurement may have discrepancy according to cardiac cycle and definition for edge. The aim of this study was to define the normal values of aortic root size in normal Korean population and to determine which guideline for echocardiographic measurements can minimize measurement variability. Methods: Healthy Korean adults were enrolled. The aortic root diameters were measured twice at 4 levels (aortic annulus, sinuses of Valsalva, sinotubular junction, and ascending aorta) by the 2005 American Society of Echocardiography (ASE) guidelines (measured in diastole, using the leading edge to leading edge) and 2010 ASE pediatric guidelines (measured from inner edge to inner edge during systole), respectively. Results: One-hundred and twelve subjects were enrolled (age 20-70). The aortic diameters obtained by 2005 ASE and 2010 ASE pediatric guidelines were as follows: aortic annuls, 2.3짹0.2 and 2.1짹0.2; sinuses of Valsalva, 3.2짹0.4 and 3.1짹0.3; sinotubular junction, 2.6짹0.3 and 2.6짹0.3; ascending aorta, 2.7짹0.4 and 2.9짹0.4cm. The diameters of sinuses of Valsalva, sinotubular junction, and ascending aorta increased with age, but that of aortic annulus did not vary with age. Intraobserver variability was similarly good in both measurement methods, but interobserver variability was slightly lower in 2005 ASE guidelines (r=0.43-0.94). Conclusions: This study detailed the normal values of aortic root dimensions in Korean adults. Two guidelines for echocardiographic measurements used in this study exhibited similar intraobserver and interobserver variability with slight differences according to the part of aortic root measured.