When using synthetic fiber rope as a mooring line of a floating body such as floating offshore wind turbine (FOWT), it is necessary to carry out characteristic test and to grasp well about strength, stiffness, durability against monotonic and cyclic loadings. In this research, we have made characteristics test of polyester rope based on ISO. Next, based on the obtained characteristic values (mass, stiffness, strength, etc.), the dynamic response analysis of the floating body-mooring system was carried out and the mooring design was carried out. It was actually operated as a floating body mooring line for about 1 year. During the operation period, no abnormality was found, nor appearance damage occurred. After completion of operation for 1 year, the polyester rope was collected and residual strength test was carried out. As a result, no serious deterioration situation such as infiltration of marine organisms or fracture of the strands due to wear between fibers was observed at all. On the other hand, with respect to durability, it was found that the strength reduction was 2.9% from the initial state with respect to the breaking strength.