The aim of this paper is to implement different grounding system (GS) models in order to compare its influence on the calculation of overvoltages in transmission lines, produced by direct lightning strikes. The models studied were a lumped DC resistance, a non linear resistance, and a synthesized circuit based on the frequency response of the GS. The latest model was proposed by the authors previously and takes into account the frequency dependent transient behavior of the GS. The overvoltages simulations were carried out by means of the ATP/EMTP program.