This paper introduces the design method for the practical use of a wave energy converter (WEC), and the associated results are application to the commercially available WEC for buoys. Peak performance of WEC occurs at resonance with driving waves. This type of resonance occurs when one of the parameters in an oscillator varies periodically. The water column in a WEC oscillates under the effect of gravity and the compression of an air chamber. The analysis of WEC is developed by assuming independence of the buoy heaving motion and the motion of the water column within the center cylinder. Results of analysis are then compared with simulation data, and applied to designing a WEC for buoys. Also, the effect of the various parameters such as cylinder length, period, mass and wave height is analyzed for the optimum design of a WEC. Finally, the research results are applied to a wave simulator with operating LabView, and some ideas are presented to the design method of WEC for buoy with simulation experiment.