The modal damping evaluation method is investigated for structural cables which diameters are comparative large with the length and which sag ratios are comparatively small. An energy-based representation of modal damping is theoretically derived considering both axial and bending stiffness. It is shown that the modal damping is expressed by parameters ξ concerning cable stiffness in transverse direction and λ concerning cable sag. It clarified that the modal damping tends to be influenced of bending stiffness when these parameters are small. Moreover, the approximation evaluation formula is proposed. As a result of applying to the cable-stayed bridge cables, it is confirmed that this method is practical by giving a suitable loss factor.