We reduced the gate tunnelling current by seven times and suppressed NBTI using plasma nitridation-induced re-oxidation (PIROX). In plasma nitrided gate oxynitride, the nitrogen concentration at the MOS interface is determined after plasma nitridation process, which affects the electrical and physical properties of gate oxynitride. To facilitate the control of nitrogen concentration at the MOS interface, an additional reoxidation process is needed, but decreasing the nitrogen concentration. In this paper, the plasma nitridation process is proposed that realizes simultaneously the nitridation and re-oxidation without an additional process and the decrease of nitrogen concentration. The control of nitrogen concentration and the amount of re-oxidation under high pressure process improves the gate tunnelling current, mobility, and NBTI.