The initial interface and silicide formation induced by Mg adsorption on the Si(111)7×7 surface have been studied using low-energy electron diffraction, x-ray photoelectron spectroscopy, and synchrotron radiation photoelectron spectroscopy. At room temperature, it is found that Mg atoms are preferably adsorbed on top sites of Si adatoms and rest atoms on the Si(111)7×7 surface and with increasing of Mg deposition, a Mg 2Si epitaxial layer is formed and the surface structure transforms from the diffuse (1×1) phase into the (2/3√3×2/3√3) R30°. After growing up to a critical thickness, the Mg film grew in a disordered phase on the epitaxial layer. The Fermi level of the Mg2Si film is positioned at 0.51±0.05 eV above the valence band maximum. On the other hand, at 300°C the Mg2Si epitaxial layer was formed in the (1×1) phase on the Si(111)7×7 and grew up to a critical thickness in the initial stage. For the successive evaporation, the Mg film grew in a disordered phase on the Mg2Si(111)1×1 surface.