The motion response of a high speed SES in waves is important because the ride quality of passengers is mainly affected by it. The pitch motion has a large influence on the vertical motion at the bow. But the pitch motion of SES does not have been analyzed properly. The reason for that is the absence of proper mathematical model for the stem bag, the bow seal, and the inherent non-linearity. In this paper, the heave and pitch motion of high speed SES in waves have been treated. For doing it, the mathematical model for the stern bag was set up, and the hydrodynamic forces on the side hulls were obtained by using the principle of momentum change. The motion responses in waves were calculated, and the analysis of the motions was done.