Abstract: Due to the opening or closing of circuit breakers and disconnect switches in Gas Insulated Substations (GIS), especially in the pumped storage power stations, Very Fast Transient Over-voltages (VFTO) are generated. This paper describes the 500 kV and 750 kV GIS. The variations of VFTO magnitudes at different points in 500 kV and 750 kV GIS during different switching operations have been calculated and compared by using Mat lab/Simulink. In this paper the effective factors on the level of VFTO is investigated and the beneficial approaches for the industry to finding the optimum approaches for VFT mitigation is presented. These factors are included residual charges, resistance, spark resistance and entrance capacitance of transformer.