The interface and silicide formation on Si(111)7x7 surface at 300 degrees C by Mg deposition was studied using low energy electron diffraction and synchrotron radiation photoelectron emission spectroscopy. At 300 degrees C only Mg/Si(111)1x1 structure was observed, which is different structure from the Mg/Si(111)1x1 surface formed at room temperature. A disordered Mg film was formed for further Mg deposition. We observed that the thin silicide layer between Mg film and Si substrate play a role as interdiffusion barrier for Mg atoms.