It is supposed that wave loads exerted on a ship advancing in large amplitude waves show quite different from those in small amplitude waves due to the fact that the submerged portion of ships hull varies remarkably with time, and coupling phenomena between longitudinal and horizontal hydrodynamic forces may occur, and etc. To develop a theoretical, but practical, method of predicting wave loads caused by large amplitude waves, and of clarifying the nonlinear characteristics is the most important goal of the present study. In this paper, 1st report, wave loads acting on a ship fixed in large amplitude waves is treated. All the results of the wave loads computed by the present method show good agreement with experimental ones in predicting the peak values of wave loads as well as their time dependent nonlinear behaviour. The main advantage of present method based on the conventional Ordinary Strip Method is that estimation of nonlinear wave loads in large amplitude waves can be easily performed with sufficient accuracy without solving three-dimensional and/or higher order potential problem.