In this paper, the authors investigate into vertical motions of a high-speed boat travelling in head sea and wave loads acting on it, taking account of the effects of non-linear hydrodynamic forces and dynamic lift in waves. A series of tests was carried out to measure heave and pitch, bow acceleration, vertical bending moment and shearing force at the midship section of a segmented model. The experimental results were compared with theoretical values predicted in the following three different ways : (A) linear calculation, that is to say, the Ordinary Strip Method applied to the running condition at travelling in a calm water, (B) non-linear calculation in which the effects of time-varying sectional hydrodynamic forces were taken into account, and (C) non-linear calculation taking account of the effects of planing at a high-speed as well as the effects of time-varying sectional hydrodynamic forces. As the result of comparison, it was shown that vertical motions and wave loads of high-speed boats can be evaluated by the method presented in this paper with accuracy enough for practical use, and that the method may be applied to the case of travelling at a higher speed than that considered in this paper if the sectional hydrodynamic forces will be appropriately evaluated when travelling at that speed.