In order to utilize the offshore wind energy, the wind turbine of a floating type is expected for its realization. This paper presents motion analysis of a floating offshore wind turbine due to wind loadings considering the effect of rotor-rotation. The horizontal axis downwind turbine is mounted on a floating foundation of a spar-type. The wind forces acting on the blades are calculated by the blade element momentum method. In order to account for the effect of finite amplitude motion of the floating body and the rotor-rotation, the multibody dynamics theory is employed. The dynamic analysis is made in time domain. The numerical results have shown that the transverse and rotational motion around the axis of the tower is generated due to the effect of gyro-moment of the rotor-rotation.