It is a general consensus that high-k dielectric films, transition metal oxides or rare earth oxides, have to be introduced for future generations of CMOS technology. However, high-k gate dielectric materials are found to have many inherent reliability problems because of their fundamental material properties and the instable Si/high-k interface. Particularly, the thermal instability, the poor interface properties with silicon, interface silicate layers formation, high interface and oxide trap density, low breakdown field and low mobility have become major concerns on the reliabilty of the MOS device. This work highlights the issues related to the thermal instability of high-k materials. The instabilties associated with high-k dielectric/Si interfaces will be also discussed.