This study conducts simulation of a 420 kV GIS substation in order to study on Very Fast Transient Overvoltage (VFTO) caused by disconnect or Switches operation. All components and its connected equipment in this GIS substation are modeled in EMTP-RV software. The various disconnect or switch operation conditions are considered and the results are presented. Finally, the effect of trapped charge on VFTO magnitude is discussed.