Abstract: Nonstoichiometric binary metal diboride superconductors Mg₁+_xB₂ (x=0,0.1, 0.2) are prepared by solid state reaction method. The powder X-ray diffraction patterns indicate that the excess amount of Mg lead easily to the insulating MgB₆ and MgO impurities and the solubility of Mg in MgB₂ is limited. The resistivity in normal state increases rapidly with increasing x.