This paper introduced characteristics of the offshore waves and vertical buoy motions observed by the two sets of the GPS buoy installed at the Northern Pacific Coast of Japan. Wave heights are computed by observed vertical buoy motions obtained RTK-GPS system. However, the vertical response of buoy motions are not taking into account in the computation wave height. Authors show observed wave height is not correct when significant wave periods less than 6.0 s. Then, in this study,modified wave height is proposed taking into account the wave response of vertical buoy motions when significant wave height are less than 6.0 s.
