This work investigates the work function modulation of TaNx films and the thermal stability of Cu/TaNx stack as a gate electrode for metal-oxide-silicon devices. The N/Ta ratio was varied in the range of 0.30-0.65 by using reactive-sputter deposition with various Ar/N2 mass flow ratios. The TaNx films are almost amorphous and are thermally stable up to 800°C. However, the formation of Ta3N5 phase in a film with a high N/Ta ratio or annealed at high temperature increases the resistivity. The work function of TaNx is about 4.31-4.38 eV and the modulation is less than 70 mV. Such a short range modulation of the work function implies than TaNx is only suitable to be a gate electrode of surface channel n-channel metal oxide semiconductor field effect transistor (NMOSFETs). The mean value of the flatband voltage decreases and the deviation of the flatland voltage increases with the increase of the annealing temperature. Although phase change, grain growth, and Cu contamination contribute to the instability at high temperature, thermal stress-induced oxide charges dominate this decrease and deviation of the flatband voltage at temperature below 500°C. According to the material and electrical analysis, the Cu/TaNx stack gate electrode can be used for NMOSFETs only, and the maximum process temperature following gate electrode deposition should be lower than 500°C.