Model experiments were carried out for a container ship in long crested irregular waves to investigate the non-linear effect on ship responses such as ship motions, wave induced pressure and wave load varying with wave height. The wave condition was changed from moderate to rough sea state in head waves. Numerical computations, by means of the strip method, the slender ship theory and the non-linear time domain strip method, were also carried out. The calculation results were compared with the experimental data to confirm the accuracy and to check on the range of the applicability. As a result, non-linear effect on vertical bending moment and wave induced pressure were remarkably observed in large waves : with increase of wave height, the bending moment acting on fore part is accelerated due to the influence of slamming and the induced pressure at the position near water surface is repressed due to the influence of exposure to the air. A time domain strip method can predict well the significant/maximum value of non-linear vertical bending moment in large waves. Toki's correction method is useful for improving the calculation accuracy of the induced pressure in regular and irregular waves.