(100) Si was oxidized in N2O ambient, and the film properties of oxynitride dielectrics were compared with pure SiO2. The growth rate, after pre-oxidation in O2/N2 ambient with raising temperature, is faster than that of O2/N2O treatment during the same condition. Nitrogen piles up at the interface of SiO2 and Si substrate and the content is about 2atom%. Comparing with pure SiO2, oxynitride dielectrics shows less dielectric breakdown failures and flat-band voltage shift, and good diffusion barrier property to dopant(BF2) is also observed.