To have an understanding of why and how slow drift damping forces of floating structures increase in waves, an experimental study is carried out using forced oscillation techniques. The object of this study is set on the viscous damping force. Two types of ocean structure are selected as the model, Tension Leg Platform and Semisubmersible. In case of TLP model the viscous drag force acting on columns (vertical cylinders) occupies most of the drag force on the total model, on the other hand, the drag force on lower hulls (horizontal cylinder) of semisubmersible is given a relative importance. These models are selected because the viscous drag forces acting on the models are dominant in the slow drift damping force and the differences in the characteristics of the drag forces acting on the vertical and horizontal cylinders should be made clear. With the view of investigating the damping forces for the slow drift oscillation, the viscous drag forces acting on models forced to surge in combined two harmonic motions in low and high frequencies are measured and those on the models forced to surge with large amplitude and low frequency in regular waves are also measured. In the experiments on the TLP model, forces acting on the columns and lower hulls are measured separately. The drag coefficients obtained from these experiments are compared with those obtained from the simple harmonic forced surging tests. Experiments show that the drag coefficients of models performing the slow drift oscillation in waves depend on the reduced velocity Ur and in the large Ur region they are higher than those obtained from simple harmonic oscillation tests in still water. The rate of increase in the drag coefficient of the horizontal cylinder is much higher than that of vertical cylinder. By this reason the rate of increase in the slow drift damping force acting on the semisubmersible is greater than that for TLP model.