How much the volume of wave overtopping and potential energy are gained has been investigated experimentally by propagating two-dimensional regular waves and irregular waves to a structure. The structure was composed of two parts. One is a sloping wall to promote the wave-overtopping and gain effectively the difference of water head. The other is V-shaped vertical walls to converge widely distributed wave energy. A relationship between the overtopping volume, head difference and the structure form has been obtained empirically. It enables us to determine the optimum form of a man-made structure to gain the required wave-overtopping volume and head difference.