Background: Assessment of myocardial function is essential in heart disease, but in regard to systolic and diastolic functions such evaluation has limitation. Ejection fraction is difficult to assess in abnormally-shaped ventricles, and diastolic inflow velocity pattern may be fused because of tachycardia. Objective: A myocardial performance index (MPI) or Tei index has been developed for adults and children. It is a Doppler derived non-geometric measure of ventricular function and is independent of heart rate. This index measures the ratio of isovolumic contraction time plus isovolumic relaxation time to ventricular ejection time. Methods: We studied 108 healthy children from 3 days to18 years of age, in whom ejection fraction was measured using M-mode (73±8%) and 2-dimensional echocardiogram biplane Simpson's method (62±7%). Results: Right and left ventricular myocardial performance indices were 0.25 ±0.09 and 0.36 ±0.11 respectively. No correlation was found between Echo Doppler index and age, heart rate and left ventricular dimensions. Conclusion: Thus, MPI is a simple and accurate tool for quantitative assessment of right and left ventricular functions and because of easy application and reproducibility; it could be regarded as an important measurement in a comprehensive hemodynamic study, especially in those with abnormal ventricular geometry.