This paper presents the theoretical analysis of the variations in the drift velocity and ionization coefficient of electrons in the presence of an insulator in gases at high pressure. The model is developed taking into consideration the variation of parameters like drift velocity, ionization coefficient, transit time of travel of electron and streamer length in the presence of insulator which are responsible for flashover of insulator in gases at a reduced voltage unlike the plain gas gap conditions. Some of the parameters calculated theoretically are found to be in close agreement with the existing experimental data. © 2013 IEEE.