Abstract: Ultrathin SiO₂ dielectric layers of thickness less than 10nm on silicon substrate were prepared by dry oxidation. For these oxide layers, rapid thermal nitridation (RTN) was performed. Here, X-ray photoelectron spectroscopy (XPS) and surface charge spectroscopy (SCS) were employed to study the nitrogen distribution in the oxynitride layers and the change in the interface state density (D_it) due to the nitridation incorporation. It is found that while most of the incorporated nitrogen are located near the oxynitride layers/Si interface, the D_it slightly decreases with the result from the rapid nitridation. Besides the breakdown field deduced from the dielectric surface potential is enhanced by the incorporation of nitrogen.