We found an anomalous behavior of dopant diffusion in Si substrate during the oxynitride process. SIMS measurements showed a notably enhanced diffusion during the NO and N2O oxynitride process. The considerably enhanced diffusion was also observed in re-oxidation of oxynitride film grown by the NO annealing or NH3 nitridation of a SiO2 film. In order to simulate the enhanced diffusion, an enhancement coefficient was introduced, showing that the simulation results are in reasonable agreement with the experimental ones. We applied the diffusion model to the simulations of MOSFETs fabricated under various conditions of the oxynitride process. The device characteristics of MOSFETs were successfully reproduced by adopting a suitable dependence of nitrogen concentration CN on the surface recombination rate of interstitial Si at the oxynitride/Si substrate interface.