Very fast transient overvoltages (VFTO) generated in a gas insulated substation (GIS) during disconnector switching operations is one of the major concerns in insulation design of the GIS. Such over voltages can cause malfunctioning of the protection and control circuits in addition to initiating faults inside the gas insulated bus ducts of the substation especially in the presence of metallic particles. In this paper, VFTO have been estimated at various points in a 1000 kV rated substation for all possible valid disconnector switching operations. For the substation studied, the maximum computed overvoltage is 1.58 pu without considering the trapped charge on the busbar. Major frequency components in the simulated VFTO lies in the range of 840 KHz to 30 MHz. Simulations have been carried out using the Electromagnetic Transient Program (EMTP).