We investigate the difference of flatband voltage (Vfb) behavior in high-k/SiO2/Si stack structure due to oxygen vacancy (Vo) and additional oxygen generated by the reduction and oxidation annelaing processes, respectively. The Vfb of Mg and La-incorporated Hf-based high-k dielectrics is also influenced by Vo generation in high-k layer. We found that the non-linear relationships of Vfb behavior appears in HfSiOx, Mg 2+-HfSiOx, and La3+-HfSiOx dielectrics as a function of the oxidation annealing temperature, while the HfO2, N3+-HfSiOx, Mg2+-HfO 2, and La3+-HfO2 dielectrics show the linear relationships of Vfb shift by introducing additional oxygen. Furthermore, it is clear that the Vfb shift of all high-k materials satisfies the diffusion equation; which indicates that the oxygen transfer in high-k layer is a dominant factor in determining Vfb. We found that the oxygen diffusion in high-k materials can be ordered as follows: Mg2+-HfO2, La 3+-HfO2 and N3+-HfSiOx > HfO 2 >> La3+-HfSiOx > HfSiOx and Mg2+-HfSiOx. Note that the oxygen transfer in high-k materials is very important to recognize mechanism of Vfb shift for high-k/SiO2/Si stack structure.