In this paper, subthreshold swing characteristics have been presented for double-gate MOSFETs, using the analytical model based on series form of potential distribution. Subthreshold swing is very important factor for digital devices because of determination of ON and OFF. In general, subthreshold swings have to be under 100mV/dec. The channel length $L_g$ is varied from 30nm to 100nm, and channel thickness $t_{si}$ from 15 to 20nm according to channel length, and oxide thickness 5nm to investigate subthreshold swing. The doping of channel is fixed with $10^{16}cm^{-3}$ p-type. The results show good agreement with numerical simulations, confirming this model.