The effects of radio frequency (RF) source power for decoupled plasma nitridation (DPN) process on the electrical properties and Fowler-Nordheim (FN) stress immunity of the oxynitride gate dielectrics for deep nano-technology devices has been investigated. With increase of RF source power, the threshold voltage (Vth) of a NMOS transistor(TR) decreased and that of a PMOS transistor increased, indicating that the increase of nitrogen incorporation in the oxynitride layer due to higher RF source power induced more positive fixed charges. The improved off-current characteristics and wafer uniformity of PMOS Vth were observed with higher RF source power. FN stress immunity, however, has been degenerated with increasing RF source power, which was attributed to the increased trap sites in the oxynitride layer. With the experimental results, we could optimize the DPN process minimizing the power consumption of a device and satisfying the gate oxide reliability.