Objectives To evaluate the values of notched on R wave in inferior leads of electrocardiogram in infant with secundum atrial septal defect. Methods To observe and compare the prevalence of notch on R wave in inferior leads combined with incomplete right bundle branch block (IRBBB) in 162 cases with secundum atrial septal defect (group I) and 162 cases without heart disease (group II). Results The prevalence of notch on R wave in all the three inferior leads, in at least one inferior lead with IRBBB and in all the three inferior leads with IRBBB were 27.16%, 14.20%and 10.49%respec-tively in group I, and were 3.09%, 1.85%and 0.62%in group II respectively, and the differences between two groups were statistically significant (P<0.005). The specificities of notch on R wave in diagnosis of atrial septal defect were 96.91%, 98.15% and 99.38% respectively. Conclusions Notch on R wave in inferior leads is an independent electrocardiographic sign of secundum atrial septal defect in infants, and thus can be used as a diagnostic parameter.