This paper describes experimental studies on the horizontal impact of flat plates falling onto a still water surface with a trapped air condition. In order to make clear the effects of the mass of falling body, plate breadth, and impact velocity on the maximum impact pressure, we have carried out the experiment of plate breadths of 0.2, 0.4, 0.6 m, in addition to those of 0.25, 0.35 and 0.5 m done in 1983.The results of the data analysis confirmed the following effects ; (1) The maximum pressure rises in proportion to the parameter K _m , K _m =1/ (1+ M _w / M ) where, M is the mass of the falling plate and M _w is its added mass of the water, (2) The impact duration depends on B / V ₀, where, B is the breadth of plate and V ₀ is the impact velocity. Based on these facts, we have developed the following empirical formula to predict the maximum pressure and the impact duration Δ t , P _max= M _w K _m V ₀/ B Δ t M _w =π/8ρ _w B ² 1/Δ t =0.144 × 10³ ( V ₀/ B ) ^0.673 where, V ₀ in m/s, B in m, Δ t in s. The agreement between experimental and predicted results is remarkably good. The formulae developed including the evaluation of the impact duration are suitable for the design of structures exposed to water impact.