The optimal timing for pulmonary valve replacement after Tetralogy of Fallot (TOF) repair remains controversial. In this study, we estimated the feasibility of using flow energy loss (FEL) to predict right ventricular (RV) deterioration due to pulmonary regurgitation after TOF repair. We examined RV outflow tract (RVOT) flow in nine patients who underwent TOF or double-outlet right ventricle repair in the intervention group (Group I) and compared them with three healthy children in the control group (Group C). We evaluated flow across the RVOT and pulmonary valve by vector flow mapping (VFM) on echocardiography and by phase contrast-magnetic resonance imaging (PC-MRI). Next, we calculated FEL and analyzed the relationship between FEL and clinical parameters of RV function. The mean FEL was significantly greater in Group I than in Group C (p = 0.002). Flow pattern and FEL were comparable by VFM and PC-MRI at the same phase 14.6 years after TOF repair. There was a significant positive correlation for the cardiothoracic ratio with both the mean FEL [correlation coefficient (r) = 0.78; p = 0.012] and the diastolic peak FEL (r = 0.75; p = 0.021) in Group I. There was also a significant positive correlation between the serial change in QRS duration with both the mean FEL (r = 0.82; p = 0.014) and the diastolic FEL (r = 0.70; p = 0.052) in Group I. FEL by VFM is an effective tool for evaluating ventricular deterioration caused by RV workload.