Comprehensive experimental studies regarding ship motions and wave loads using two different types of merchant ship models : a VLCC model and a large-container ship model, have been conducted respectively in order to investigate the nonlinear characteristics with respect to waveheight of motions and wave loads in large waves. Six degrees of motions, vertical as well as horizontal bending moment and hydrodynamic pressures were measured in a quite systematic manner in different wave encountering angles, different wave frequencies and different incident-wave heights as the ship models were freely advancing without any external restrictions. The nonlinear characteristics of the two different types of merchant ships observed in ship motions and wave loads are discussed in detail with obtained experimental results and numerical ones calculated by means of strip method. As the results of these studies, the followings may be concluded. 1) Nonlinearities with respect to waveheight are confirmed in such a way that the motion responses per unit incident-wave amplitude become smaller as the waveheight increases, which are especially remarkable in rolling and heaving motions. 2) The nonlinear waveheight characteristics of the hydrodynamic pressures are observed distinctively not only at the locations near the water-surface but also at the locations well bellow the water-surface. 3) The nonlinearities of the wave loads on the VLCC model are relatively small but were manifested in the container ship model especially in quartering seas. 4) It is characteristic that nonlinear characteristics are persistently observed in virtually any responses of the container ship model advancing in quartering head seas, which may be caused by the geometrical features of the container ship such as the large bow-flare.