In general, the salient features of the floating breakwater have excellent regulation of sea-water keeping the marine always clean, up and down free movement with the incoming and outgoing tides, capable of installing without considering the geological condition of sea-bed, and capable of installing at any water depth. This study discussed the three dimensional wave transformation of the floating breakwater moored by pier, and its dynamic response numerically. Numerical method is based on the boundary integral method and eigenfunction expansion method. It is known that pier mooring system. Pier mooring system permits only vertical motion (heaving motion) of floating breakwater, other motions restricted. It was assumed in present study that a resistant force as friction between pier and floating pontoon was not applied for the vertical motion of the floating breakwater. So, the floating breakwater moves freely for vertical direction. According to the numerical results, draft and width of the floating breakwater affected on the wave transformations greatly, and incident wave of long period was well transmitted to the rear of the floating breakwater. And the vertical motion came to be large for the short wave period.