We have characterized physical and electrical properties of the HfO2/HfSixOy thin film for gate oxides in the metal-oxide-semiconductor device. The oxidation of Hf deposited directly on the Si substrate results in the HfSixOy interfacial layer and the high-k HfO2 film simultaneously. Interestingly, the postoxidation N-2 annealing of the HfO2/HfSixOy thin film reduces (increases) the thickness of an amorphous HfSixOy layer (HfO2 layer). This phenomenon causes the increase of the effective dielectric constant, while maintaining the excellent interfacial properties: The equivalent oxide thickness and the leakage current density of the Pd-HfO2/HfSixOy-Si capacitor were 1.4 nm and 5 x 10(-3) A/cm(2) at 2 V after compensating the flatband voltage of 1 V, respectively.