Many researches on water impact pressure have been reported and the following facts are well known : a) When the impact surface is parallel to the water surface, air is entrapped between the bottom plate and water. b) On the other hand, when the impact surface is slightly inclined to the water surface, a very sharp pressure sweeps along the plate. Authors carried out water impact tests using one fifth models of a forward bottom structure of a VLCC (plate thickness was considerably increased). And following results are obtained. 1) When the impact surface is parallel to the water surface, pressure time history becomes very dull by the effect of the entrapped air and the same pressure occurs in large area. Then, equivalent static pressure of each member (bottom plate, longitudinals, bottom transverses, etc) is almost same as to the maximum pressure measured by the pressure gauge. 2) In the case of impact by slightly inclined surface, a very high pressure occurs only in a very narrow band in front part of sweeping pressure, and the effective pressure to the structural response becomes much smaller compared with the peak pressure measured by the pressure gauge. The extent of this reduction in effective pressure depends on the area of structural part under consideration. 3) In the latter case, the effective pressure varies much more slowly than the pressure obtained by a pressure gauge. And it can be said that the effect described in 2) is larger than the so-called dynamic effect. Those facts should be considered at designing of a structure under impact loads obtained by model tests.