In this paper, two types of slamming impact load are investigated both theoretically and experimentally, assuming that the water entry speed is constant and gravitational effects are negligible. One is of the water impact of a wedge and the other is of the impact caused by trapped air layer between a flat bottom and a water surface. For the impact of a wedge, numerical calculation is conducted by means of the Boundary Element Method taking the splash into consideration. The results are in good agreement with Wagner's theoretical ones. In case of the impact of a flat bottom, it is clarified from the experimental results that the impulsive phenomenon is composed of two stages. The phenomenon of the first stage is one of before contact of the bottom with a water surface and the second stage is one of after the contact. For the impact during the fist stage, numerical calculation is conducted by means of the characteristic curve method assuming that the flow of the air layer is onedimensional compressible one. For the second stage, the impact pressure is obtained by solving a system that is composed of the bottom, the a!r spring and the added mass. The numerical results and experimental ones agree precisely in the first stage, but in the second stage the calculated values are higher than the experimental ones.