For relief of right ventricular outflow tract obstruction in the operative treatment of tetralogy of Fallot and other complex congenital heart disease, it is often necessary to perform transannular monocusp patch to prevent right ventricular pressure overload and reduce pulmonary regurgitation. But the the geometric relationship between a monocusp patch length and the size of RVOT is not well defined. Five 20 mm sized monocusp patches were tested in simulated RVOTs which sized from 18 mm to 22 mm at 1 mm interval, respectively. The hydrodynamics of the patches were assessed through a quasi-physiological artery pulsatile flow duplicator system. The transvalvular pressure difference, effective orifice area and regurgitation flow were determined. The results showed that the regurgitation fraction increased with the diameter of RVOT increased (from 6.25% at 18 mm RVOT to 26.63% at 22 mm RVOT). Implant an oversized monocusp patch in RVOT reconstruction can effectively decrease the regurgitation but increase the transvalvular pressure and may reduce the longevity of leaflet.