Pulsating valvular flow in a 3:1 enlarged bileaflet mechanical heart valve (MHV) model was investigated using a two-dimensional digital particle image velocimetry (DPIV) technique. Strong interactions between the fluid and the moving leaflet were characterized by instantaneous velocity vector plots. The hinge position, indicated by d/W (d = distance between the two axes of the hinge pivots; W = width of the testing section in the middle plane), played a critical role in MHV performance. Extreme low d/W caused temporal stagnation of flow in the central orifice, while a high d/W tended to produce instability and highly disturbed fluid motion which led to abnormal opening and closing behaviors. There exists an optimum hinge position with d/W in the range 0.2-0.3. This information may be useful for future MHV design.