In this paper, a method to calculate the vertical hydrodynamic forces acting on a high speed vessel with vertical motions is described. It is basically based on the Chapman's formulations. The radiation condition is treated by following the same method of Yamasaki & Fujino. In which, a more relaxed Sommerfeld's radiation condition is imposed on an artificial open boundary. As an application of the method, the diagonal and cross-coupling added mass and damping of heave and pitch for a high-speed boat as well as fishing boat are calculated and compared with the experimental results prepared by the technical committee on “Study on Seakeeping Qualities of High Speed Vessel” (SSH). Besides, calculations following a nonlinear O. S. M. are also conducted for comparison. As the result of comparison, it was shown that the vertical hydrodynamic coefficients of high speed vessels can be evaluated by the method presented with relatively better accuracy in general.