High-permittivity (high-k) dielectrics with HfO2/Hf xSi1-xOy/Si structures were fabricated using plasma oxidation and subsequent annealing of Hf/SiO2/Si structure. By replacing SiO2 film of initial structure from plasma oxidized SiO2 to thermal oxidized SiO2, the drastic decrease of traps in interfacial layer (IL: HfxSi1-xO2y) could be successfully achieved, which shows interface state density of 1 × 1011 eV-1cm-2, effective oxide thickness (EOT) of 1.2 nm, and 4 orders decrease of leakage current density relative to SiO2 with EOT of 1.2 nm. The influence of post annealing on structural and electrical properties of IL was investigated by using XPS analysis and TEM observation. It was clarified that the increase of EOT after post annealing at 900°C is caused by the decrease of Hf content in IL and the increase of IL thickness. The kinetics of IL formation is discussed in details.