A series resistance model is developed to accurately predict the drain/source resistance of MOSFETs in the ultra submicron regime. The series resistance model is based on the differential resistance concept and the mean value theorem of integrals. Three parameters of the model may be obtained through the multiple linear regression method. In this study, we employ numerical simulation data to fit three parameters for the planar nMOSFETs and nLDD-MOSFETs, of which the substrate doped concentration is $1\times10^{15}\sim~1\times10^{16}{\rm~cm}^{-3}$, and the channel length is 45$\sim$2000 nm. We also obtain a semi-empirical formula with a maximum error of only 9.5%. The theoretical model and calculation results of its semi-empirical formula show that the drain/source resistance is only related to the depth of the drain/source pn junction, resistivity, and length from the channel to the drain/source electrode, and that the influence of the channel length, stack-gate length, and electrode length is negligible. Because the semi-empirical formula is simple, with high precision and a clear physical concept, and allows for easy parameter extraction, it can be used in a characteristic analysis of MOSFETs and circuit simulators.