Abstract: In order to evaluate the damping of carbon fiber reinforced plastic (CFRP) cables used in bridges, the free vibration of a CFRP wire has been tested, and the loss factor of bending strain energy has been discussed according to the energy-based evaluation of modal damping. By performing a test at different frequency conditions, the relationship between the damping and the vibration amplitude of CFRP and steel wires has been studied. According to the experimental results, the damping increases with the vibration amplitude. However, the variation patterns of the damping-vibration amplitude curves are observed to be different between CFRP and steel wires. The damping of the CFRP wire is smaller than that of the steel wire while the strain amplitude is similar. The energy-based evaluation is proved to be efficient for the calculation of damping of steel wire, because the energy loss factor is found to be independent of the vibration amplitude of steel wire. For CFRP wire, the tested energy loss factors are not as regular as those of the steel wire. However, the energy-based evaluation could also be used to estimate the damping of CFRP wire while the vibration amplitude is distributed in a certain range.