In order to reveal the nature of correction factor n for the wave pressures, the effect of the nonlinearity of progressive waves on underwater pressures was investigated. The generalized equations for wave pressures were derived from the wave theory of higher order approximation, and the magnitudes of harmonic components of wave presuress were determined as the functions of wave characteristics.
　The value of n ectimated from the above analysis and measured in a wave tank for regular trains of progressive waves does not exceed the value of 1.2～1.25 and even becomes smaller than 1.0 if the ratio of water depth to wavelength is larger than 0.1.
　The actual values of the correction factor n measured at coastline are 1.3～1.5, however. Such the large values of n cannot be explained due to the effect of wave nonlinearity. The statistical characteristics of actual waves are considered to govern the value of n greatly. The formation of bound waves by the interference of surface waves is also mentioned as a contributory cause for the increase of the value of n.