Ships of the small draught/length ratio may suffer from serious structural damages due to slamming in heavy sea. The strength of a ship in such conditions can be investigated by studying its motion. In this paper the authors investigate theoretically the motion and longitudinal strength of a tanker in head sea taking account of the effects of nonlinearities such as the hull shape nonlinearity, bottom emergence, and hydrodynamic impact, and a series of tests was carried out in a basin, measuring the heaving and pitching angles, the acceleration at the bow, and the bottom pressure along the base line ; the experimental results were compared with calculations, which shows good agreement. Conclusions obtained are as follows : (1) The heaving and pitching amplitudes themselves are affected only slightly by slamming ; (2) the acceleration at the bow increases due to the effect of nonlinearity ; (3) the time history of the calculated pressure and the measured are similar in shape ; and (4) the sagging bending moment becomes very large in slamming conditions in the fore body in the case of large ships.