The effects of hydrodynamic forces acting on the cold water pipe and mooring lines on motions of the OTEC platform are investigated experimentally and theoretically. The hydrodynamic forces acting on the cold water pipe and mooring chains oscillating in water are obtained by using mode methods. The computer time needed for the calculations is very short; therefore, the methods proposed in this paper are of great practical use. Calculated results of the bending moment and shearing force of the cold water pipe at its upper end and the horizontal and vertical components of the dynamic tension of the mooring chain at the mooring point are compared with those of experiments. They are in good agreement. Experiments and theoretical calculations on the motions of the moored OTEC platform show that the viscous drag forces acting on the cold water pipe and mooring chains have a great effect on the motion at its resonant period. They act as the viscous damping force for the motion of the platform at its resonant period, and reduce its magnification factor.