The purpose of this study is to examine the reduction effect of wave overtopping of an upright seawall with a triangle-shaped countermeasure and wave pressures exerting on the seawall and the countermeasure throughout hydraulic experiments. As a result, the countermeasure placed on the seawall was found to have a great effect on the reduction of wave overtopping rate. It was evident that the reduction effect is mainly governed by the wave steepness, the relative water depth and the relative crown height. Especially, the relative crown height had the most influence on the reduction effect. It was also found from the experimental results that large wave pressure acts on the countermeasure.