Flow profiles in the pulmonary veins (PVs) and arteries (PAs), Fontan conduit (conduit), and superior vena cava (SVC) were examined using phase-contrast magnetic resonance imaging. The relationship of the ratio of systolic forward flow volume and diastolic forward flow volume (FV-S/D) with ventricular relaxation time constant (tau) was then investigated. According to the flow profiles in the PVs, the forward flow volumes in diastole were dominant over those in systole. The FV-S/D was significantly negatively correlated with tau, not only in the PVs but also in the PAs and conduit. Moreover, tau was positively correlated with pulmonary wedge pressures, while pulmonary wedge pressures were significantly negatively correlated with FV-S/D in the PVs. In contrast, the FV-S/D in the SVC was also negatively correlated with tau, but the correlation coefficient of this relationship was lower than that of the other relationships. Lastly, the FV-S/D in the SVC correlated more significantly with the ejection fraction of the ventricle rather than with tau. On the basis of our findings, we believe that the flow profiles in the PVs are influenced by the ventricular relaxation performance. Moreover, the ventricular relaxation performance influences the flow profiles in the PAs and conduit, both of which are located upstream of pulmonary circulation. On the contrary, the flow profiles in the SVC seem to be influenced by ventricular contraction rather than ventricular relaxation.