In this paper, a full-wave approach based on the method of moment (MoM) is proposed to investigate the harmonic impedance of a tower and its connected ground electrode in the frequency domain. The accuracy of the results is validated in comparison with NEC-4. The proposed numerical method is also employed for the evaluation of a full-sized HVDC tower harmonic impedance. The main contribution is the assessment of the harmonic impedance of a real tower with detailed geometrical information connected to the multi-layer grounding system. The validity of the transmission line method is evaluated through comparison with the results computed using the developed full-wave approach at the high frequency. In addition, the simulation results assure that a real tower’s harmonic impedance could be smaller than the value estimated for very simplified models at the high frequencies. When the full-wave method is applied and the precise model of the ground electrode is considered, the harmonic impedance of the tower in the frequency domain and consequently, the transient impedance in the time domain are different, while the grounding system is assumed to be a perfectly conducting plane. These differences can become very significant, especially close to the resonant frequencies. The harmonic impedance of power transmission towers is strongly influenced by the connected grounding system.