Abstract—We observed the change of the subthreshold swings when the high-k material was used for the gate oxide of the junctionless double gate MOSFET (JLDG MOSFET). For this purpose, the analytical subthreshold swing model is proposed using the potential model derived from Poisson’s equation. The subthreshold swing derived from the model proposed in this paper is in good agreement with the subthreshold swing value from the two-dimensional numerical simulation within the error of 5%. Using this model, we observed the change of subthreshold swing with respect to the channel length, silicon thickness and gate oxide thickness with a dielectric constant as a parameter. As a result, the subthreshold swing was greatly reduced and the changing rate of the subthreshold swing by the channel length, silicon thickness, and oxide thickness was 1 mV/dec-nm or less when the material with a dielectric constant of 30 or more was used as the gate oxide. Especially, it was found that the dielectric constant of the gate oxide for the JLDG MOSFET should be more than 30 in order to have a subthreshold swing of less than 65 mV/dec.