In this study, the performance of a high-speed planing craft during rapid acceleration from rest is investigated, following series studies on the performances in steady running condition including ship motions due to instability in calm water. Measurement of running attitude of a model of planing craft during acceleration from rest is carried out to compare with that in running at constant speed. The results show the force component which is proportional to acceleration is important as well as a conventional displacement ship. Measurements of hydrodynamic forces acting on it during acceleration suggest, however, that the added mass component does not play an important role because it is much smaller than the inertia force and the steady drag components which are proportional to square of advanced speed. A computer simulation program to predict this performance is developed using a motion equation with experimental and theoretical hydrodynamic coefficients. The results of the simulation are in fairly good agreement with the experimental ones. It is shown that the procedure can simulate unsteady motions like porpoising in accelerating condition as well as in steady running condition.