Horizontal wave forces exerted on an offshore platform have been studied by three-dimensional model tests.
　The proposed formulae of horizontal wave force are expressed as the sum of a component proportional to the Froude-Krilov's force and the drag force component.
　The correction factor CF to the Froude-Krilov's force and the drag coefficient CR are evalulated for a rigid square platform according to the three-dimensional tests results. In present analyses, Stokes' third order wave theory is used to estimate wave profile and water particle velocity components.
　It may be concluded that the horizontal wave forces on a rectangular rigid platform placed near the water surface can be evaluated by using the proposed method within an error of 20%. Percentage of error is increased in case of large clearrances compared to the wave crest. It should further noted, that the method is not applicable if the ratio of platform length to the wave
　length (lD/L) is near unity.