Compared with 500 kV EHV GIS substation, 1000 kV UHV GIS/HGIS substation has different characteristics in terms of switchgear, system structure, insulation margin, requirements on safety and stability and importance. And the hazardness of very fast transient overvoltage (VFTO) on UHV GIS is much heavier than that in 500 kV power system. This chapter first discusses the basic knowledge of VFTO, and studies the VFTO and the influence factors of 1000 kV GIS substation under different operation conditions. Besides, it compares the VFTO between 500 kV and 1000 kV GIS substations, as well as between substation and power plant. It also carries out research on restriction of VFTO wave front steepness invading port of the main transformer on the overhead line. Finally, it analyzes the GIS transient enclosure voltage (TEV) in GIS substation and power plant.