Cardiac mensuration by the roentgenological heart volume method was carried out on 322 Korean school children, 196 cases of male and 126 cases of female. Formula used in this study are as follows. Predicted heart volume=K $ imes$ Broad diameter $ imes$ Long diameter $ imes$ Greatest horizontal depth. Relative heart volume={{{{{{{{ { Predicted haert volume (volume in ml)} over {Body surface area (Meter squared) } }} }} }} }}{{{{ { Predicted haert volume (volume in ml)} over {Body surface area (Meter squared) } }} }} The constant K represents the product of an ellipsoid area factor and a correction factor for magnification of cardiac image. Therefore, K value used in this study is 0.42 for 200cm and 150cm of focal film distances in posterior-anterior and lateral views respectively. Body surface area is obtained by use of Du Bois Nomogram. The results were as follows: 1. Predicted heart volume at 6 years to 10 years of age is 191.76$pm$4.66ml in male, 183.36$pm$3.64ml in female and at 11 years to 15 years of age 320.62$pm$10.34ml in male, 284.22$pm$4.79ml in female. 2. Relative hart volume at 6 years to 10 years of age is 219.88$pm$6.19ml/$m^2$ in male, 211.43$pm$7.93ml/$m^2$ in female and at 11 years to 15 years of age 258.39$pm$6.85ml/$m^2$ in female. 3. In comparison with American standard value by Mannheimer, Korean school children show smaller heart volume generally. 4. Roentgenological mensuration of the relative heart volume is considered to be simple and the best method to express the standard of the heart size.