We have analyzed the effective oxide thickness(EOT) of the dielectric material for which we have optimum performance and the output characteristics of the silicon nanowire transistors by replacing the traditional SiO2 gate insulator with a material that has a much higher dielectric constant(high-k) gate, materials like Si3N4,Al2O3, Y2O3 and HfO2. We have also analyzed the channel conductance, the effect of a change in thickness, the average velocity of the charge carrier and the conductance efficiency in order to study the performance of silicon nanowire transistors in the nanometer region. The analysis was performed using the Fettoy, a numerical simulator for ballistic nanowire transistors using a simple top of the barrier(Natori) approach, which is composed of matlab scripts. Our results show that hafnium oxide (HfO2/ gate insulator material provides good thermal stability, a high recrystallization temperature and better interface qualities when compared with other gate insulator materials;also the effective oxide thickness of HfO2 is found to be 0.4 nm. We have analyzed the effective oxide thickness (EOT) of the dielectric material for which we have optimum performance and the output characteristics of the silicon nanowire transistors by replacing the traditional SiO2 gate insulator with a material that has a much higher dielectric constant (high-k ) gate, materials like Si3N4, Al2O3, Y2O3 and HfO2. the effect of a change in thickness, the average velocity of the charge carrier and the conductance efficiency in order to study the performance of silicon nanowire transistors The analysis was performed using the Fettoy, a numerical simulator for ballistic nanowire transistors using a simple top of the barrier (Natori) approach, which is composed of matlab scripts. Our results was made that hafnium oxide (HfO2 / material provides good thermal stability, a high recrystallization temperature and better interface qualities when compared with other gate insulator mat erials; also the effective oxide thickness of HfO2 is found to be 0.4 nm.