Based on the studies of the two-dimensional nature of electrons in the inversion layer of the ultrathin gate oxide MOSFET, an approximate model to describe both the tunneling and inversion layer quantization effects on deep submicron MOSFET is developed. By using this model, the influence of direct tunneling and the quantum mechanical effects of electrons in the inversion layer on the MOSFET threshold voltage is analyzed for a range of oxide thickness and substrate doping concentration representative of deep submicron technology. The results indicate that both inversion layer quantization effect and tunneling effect should be considered when a MOSFET is scaled down to sub-0.1 mu m level.