As a step of improvement of the calculation of lateral motion (sway, yaw and roll) of a ship in waves, the coefficients of equations of motion were determined by the forced oscillation technique. Experiments made it possible to know the effects of frequency, advance speed and bilge keels on ship motions. The experimental values were compared with the calculated values by the strip method. For the main terms of sway, yaw or roll, except roll damping term, the results show a fairly good agreement. For the coupling terms of sway-yaw, yaw-roll or roll-sway, there are fair correspondences. For the prediction of the roll damping term, a proper method of approximation which includes the effects of viscous damping, advance speed, bilge keels and frequency of motion should be developed.