Human activity expanding in the field of ocean development demands larger and longer offshore structures. These large structures can easily deform under hydrodynamic forces and dynamic responses become serious problems even though the static strengths are enough. The stiffening of these structures needs large amount of material and is not feasible from the viewpoints of economy. Suppression of the dynamic response by the control technique is supposed to play an important role in the future ocean development. Furthermore the designs presupposing the control of dynamic response are expected to be realized. As a preliminary trial to control the dynamic response of the offshore structures, two simple tower-like offshore structures are chosen and the dynamic responses in the regular waves are controlled experimentally in the wave tank. One is a simple rigid pipe type structure supported at the lower end by pin joint. The other is a beam type structure fixed at the lower end. The formulation of the optimal control shows that the control is divided into two parts. One is the closed-loop control where the force proportional to the errors between the present state variables and their target value is applied to the systems. The other is the open-loop control based on the definite wave forces. The spectrum of the ocean waves in the steady state is a function of wind velocity and the phase of element waves are decided by analyses of the surface elevation. So the wave forces acting on the offshore structures can be handled as the definite forces. In this research, two types of open-loop control are employed apart from the optimization. One is the type to compensate the whole wave forces acting on the structure. The other is the type to compensate the major Fourier component of the fluctuating wave forces by using the three states of the thruster, forward, stop and backward. The displacement and deformation of the structures were measured by using ultrasonic measurement system. The surface elevation was measured by a capacity type wave height meter. These data were sampled and processed by 16 bits microprocesser and the thrust was realized by screw propeller type thruster. The performance of the control was satisfactory. The response of the controlled structures was roughly 30% of the uncontrolled system.