Thermal stability of the Zr O2 Zr-silicateSi structure and the Zr-silicide formation were investigated by photoemission spectroscopy depending on the annealing temperature in ultrahigh vacuum. By the annealing below 860 °C, the interfacial layer thickness of the Zr-silicate decreased although the Zr O2 top layer was not affected. The annealing at 860 °C caused the interfacial Zr-silicate layer to disappear. By the annealing above 860 °C, the metallic Zr components appeared and the metallic clusters were formed. High-resolution photoemission spectra have revealed that the clusters consist of a Zr Si2 layer. Valence-band spectra depending on the annealing temperature provide us with the information about the crystallization in the Zr O2 layer.