An analytical and experimental study of the side force and yaw moment on a ship model towed in severe quartering waves has been conducted. In order to investigate the side force on the ship with heave and pitch motions in a very low encounter frequency, extensive model tests have been carried out. The side force is divided into Froude-Krylov force, diffraction force and lift, and discussed in detail. The effects of heave and pitch motions to side force on the ship are investigated by taking into account the change of submerged sectional area. Comparisons of the experimentally determined force on the model and a theoretical prediction are presented.