The interfacial layer (IL) formed at the HfO2/Si interface was investigated by using Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Differently with the previous reports, it is concluded that the inner oxygen from bulk film predominates the oxidation process in interface region rather than the oxygen introduced from outer environment. First, from FT-IR, it is found that the formation of the IL strongly relies on the annealing temperature but does not obviously rely on the HfO2 thickness and the annealing atmosphere. Second, the contradistinctive images of Hf/Si annealed in oxygen ambient and Hf/SiO2 annealed in vacuum were investigated by TEM, which confirms the conclusion obtained from FT-IR data that the IL is formed not by a diffusion of the oxygen from the annealing atmosphere, but by a reaction within the interface region. Third, the Hf 4f, O 1s, and Si 2p core-level energy states of Hf/SiO2 stack annealed in vacuum were investigated by XPS using two ways, one is investigating the samples annealed in vacuum for varied time and the other is investigating the fully oxidized sample in different depth. Based on the experiments, it is concluded that the inner oxygen from bulk films (HfO2 or SiO2) has greater influence on the IL formation comparing with the outer oxygen from environment.