Objective: We investigated the cardioprotective effects of two different potassium concentrations in crystalloid cardioplegic solutions in pediatric patients undergoing tetralogy of Fallot (TOF) repair under cardiopulmonary bypass (CPB). Methods: Eighty seven pediatric patients with Tetralogy of Fallot (TOF) were randomly allocated into two groups according to the type of cardioplegic solution. Group L received large volume with low potassium concentration (K+ 10 mmol/L) cardioplegia; 30 ml/kg for induction of arrest and repeated every 20 min at a dose of 15 ml/kg. Group S received small volume with high potassium concentration (K+ 30 mmol/L) cardioplegia; 10 ml/kg for induction of arrest and repeated every 20 min at a dose of 5 ml/kg. Results: Group L showed earlier return of cardiac rhythm (33.8 ± 4.9 sec) compared to 38.9 ± 5.6 sec in group S with most of the cases had sinus rhythm. The maximum Inotropic Score in the first 24 h was lower in group L; 13 (5) compared to group S; 15 (10). Less increase in cardiac troponin I (cTnI) in all postoperative readings, shorter duration of mechanical ventilation, ICU length, and hospital stay in group L. No changes in hemodynamic parameters between both groups. Conclusion: Better myocardial protection in pediatric cardiac patients perfused by cardioplegia with low potassium concentration.