The evaluation of the geodynamic stability of rubble mound breakwater such as sliding and settling requires the information of wave forces acting on the breakwater. In the present study, a numerical method which can be practically used in determining wave forces was presented, and was verified by hydraulic experiments. The strongly nonlinear wave deformation and resulting wave pressures on the frontal slope of the breakwater were calculated by a volume-of-fluid method. The peak values and phases of pressure signal were shown to be linearly dependent on the surf similarity parameter. The results agreed quite well with experimental data, and their experimental formula were proposed. The wave forces were determined by connecting the wave pressure formulas to a boundary element method for calculating the inner wave field of breakwater. It was shown that the calculated wave forces also agreed well with experimental data.