The electrical characteristics of NiSi metal gate and their thermal stability were studied. A physical model is proposed to explain the increase in NiSi work function when the forming temperature is higher than 500 ℃. By measuring the sheet resistance of NiSi film prepared at different temperatures, it is shown that NiSi has the lowest resistance when formed at 400 ℃, which is stable from 400 to 600 ℃. The X-ray diffraction measurement for the NiSi samples formed at various temperatures revealed that NiSi phase was the main component at temperatures from 400 to 600 ℃. The capacitors formed by furnace annealing has higher equivalent oxide charge Qox and lower breakdown electric field Ebd, which proves that furnace annealing is unsuitable for NiSi metal gate fabrication due to the long time of thermal processing (400 ℃ for 30 min). The electrical characteristics of NiSi gate metal oxide semiconductor capacitors formed at various rapid thermal annealing(RTA) temperatures were studied. By comparing theC-V curves, Ig-Vg curves and Qox of the capacitors, it was found that when the RTA temperature is higher than 500 ℃, reaction between NiSi and gate oxide will occurr, reducing the quality of the gate dielectric. In conclusion, the suitable forming temperature of NiSi metal gate should be from 400 to 500 ℃. Moreover, the NiSi work function and Qox formed at 400, 450 and 500 ℃ respectively were also determined.