The optimisation of reaction temperature and time has been made in the oxynitridation of SiO2 in N2O by evaluating its dielectric properties at a constant current stress. It is found that N2O-oxynitrided SiO2 films are much improved in both charge trapping density and charge to breakdown when the optimum oxynitridation process (O2, 1100-degrees-C, 5 s and following N2O, 1200-degrees-C, 25 s) is chosen.