In order to identify the damping characteristics of transmission towers, Empirical Mode Decomposition (EMD) and Ensemble Empirical Mode Decomposition (EEMD) were employed to obtain Intrinsic Mode Functions (IMFs) of time histories of accelerations obtained from aero-elastic wind tunnel tests of transmission tower. The frequency and damping of the tower under various wind velocities and wind azimuths were identified using EEMD, improved Random Decrement Technique (RDT) and Hilbert transformer. The distribution characteristics of damping along with wind velocities in the along-line direction and in the cross-line direction were also investigated. Results show that the vibrating modes of IMFs obtained from EEMD are much clearer than those obtained from EMD. Furthermore, the mode mixing problem is still existed to some extend in the IMFs obtained from EMD. Totally speaking, the natural vibrating frequency of the tower in the along-line direction decreases with the increase of the wind velocity, whereas the natural vibrating frequency in the cross-line direction increases with the increase of the velocity. However, the trend of damping in the cross-line direction with respect to wind velocity is not obvious.