Several reconstructed structures of the Mg/Si(100) system have been observed by low-energy electron diffraction during the process of thermal desorption: (1) a (1 1) phase, (2) a (2 3) phase, (3) a (2 2) phase and (4) a second (2 3) phase. These phases appeared successively as the Mg coverage decreased. The thermal desorption spectra were measured by a quadrupole mass spectrometer. The reconstructed structures analyzed based on the Mg coverage suggest that Mg atoms on the Si(100)2 1 surface form atomic chains and the atomic arrangement on a chain changes as the Mg coverage decreases. The reconstruction mechanism on the Mg/Si(100) system is very similar to that of the Ba and Sr/Si(100) system for the group II metals of the periodic table.