The vectorcardiograms and data from cardiac catheterization of 38 patients with PDA were analyzed and compared with VCGs of VSD. The following results were obtained. (1) The patients whoes QRS loops are inscribed counterclockwise rotation in left sagittal plane have large left to right shunts. There are no relations between the inscription of QRS, T loops in another plane and th severity of the disease. (2) Large shunts results in an increase of the magnitude of the maximum posterior deflection and result in a significant posterior displacement of the maximum QRS vector in the horizontal plane. (3) The VCGs of PDA differ from the VCGs of VSD in the following vectorcardiographic findings which are closely related to right ventricular overloading. Ⅰ) Vectorcardiographic CVH patterns are appeared in 10 % of mild PDA, on the other hand, in 40% of mild VSD. Ⅱ) According to increase of the left to right shunt, the magnitude of the maximum anterior deflection of QRS loop of VSD increases, but, that of PDA does not correlate to shunts. Ⅲ) The terminal conduction delay, the maximum upward deflection and the maximum posterior deflection of the terminal portion which are caused bypertrophy of right ventricular outflow tract, have correlation with right ventricular systolic pressure in VSD, but not in PDA.