Replacement of poly-Si and SiO2 with new gate electrode and high k gate oxide is an inevitable trend for next-generation CMOS integrated circuits. Therefore, work function (£Xm) of gate electrodes as well as the thermal stability and electrical behaviors of MOS capacitors should be understood. In this study, tungsten (W) is applied as the gate electrode and the gate dielectric materials are SiO2, SiON and HfO2. £Xm of W and electrical properties of the MOS structures are investigated. £Xm,measured of W is calculated from the flat-band voltage (VFB) of MOS capacitors with dielectrics of various thicknesse. For W/SiO2/Si structure, the £Xm,measured of W is 4.67 V; however, the £Xm,measured of W in W/SiON/Si and W/HfO2/Si structures is 4.60 V and 4.84 V, respectively. The result means that the £Xm,measured of W in W/HfO2/Si structures has extrinsic contributions to Fermi level pinning. The phase of as-deposited W is £]-W (or £]-W+£\-W) phase and transfers to £\-W+WO3 mix phase after annealing at 500°C in N2+H2 ambient for 30 min. The trapped charges and oxide charges of dielectric are reduced after annealing. However, the EOT of W/SiO2/Si increases significantly after annealing, indicating the thermal stability of this capacitor is poor.