Grounding systems are considered as an essential part of power systems since they are designed to protect them against fault currents according to some safety and power quality standards. When lightning strikes a substation or a transmission tower near the substation, high current generated by the stroke would flow into the grounding system of the substation and dissipate into the soil. The transient response of the grounding system is a very important indicator to the safe and reliable operation of the power system. A model with lumped time-varying parameters is developed and is implemented into software tools (Matlab/Simulink) to calculate the transient behavior of the grounding systems during lightning strokes. The effects of different parameters especially soil resistivity and ionization of the soil around the electrode on the transient behavior of the grounding system subjected to lightning impulse current are analyzed.