An analytic model of direct tunneling current of small-scale MOSFETs in depletion and inversion is developed based on analytic surface-potential model and replacing the multi-subband with a single-subband. The simulated results are in good agreement with the results of self-consistent solution and experimental data, but take much shorter computing time than the self-consistent solution method. This indicates that the model can be used for analysis of gate-leakage properties of MOS devices with not only SiO2 but also high-k materials as gate dielectric and high-k gate dielectric stack structures, and circuit simulation of MOS devices.