Background: Critical pulmonary stenosis has been associated with severe right ventricular outflow tract obstruction (RVOTO) leading to severe right ventricular hypertrophy (RVH), impaired both systolic and diastolic functions, changes in RV shape and geometry, hence further assessment by 3D echocardiography (3D Echo) is recommended. Objective: The aim of the current work was to evaluate RV (Right Ventricle) functional indices using 3D echocardiography in infants with critical pulmonary stenosis undergoing balloon pulmonary valvuloplasty and to evaluate RV (Right ventricle) indices as follow up parameters in infants with critical pulmonary stenosis (PS). Patients and Methods: This prospective study included a total of 60 infants with critical pulmonary stenosis, their median age was 8.5 (range 2 - 12) months referred for urgent balloon pulmonary valvuloplasty to Pediatric Cardiology Unit, Department of Cardiology, Ain Shams University Hospitals. Results: 3D echo revealed lower RV (Right ventricle) volumes (P <0.001), higher FAC (fractional area change) (P< 0.001), higher TAPSE (tricuspid annular plane systolic excursion)(P< 0.001), higher EF (Ejection fraction) (P < 0.001), lower basal, mid and longitudinal RV dimension (P< 0.001). FAC (fractional area change), TAPSE (tricuspid annular plane systolic excursion) and RV basal and longitudinal diameter were significantly larger by 3-dimensional echocardiography (3D Echo) than by 2-dimensional echocardiography (2D Echo) (P=0.01). Conclusions: It could be concluded that in patients with critical pulmonary stenosis undergoing balloon pulmonary valvuloplasty, assessment of RV indices using 3 D echo is more reliable and effective method to assess RV volumes and function in comparison with conventional 2D echocardiography.