A method is described for the preparation of superconductor massive bodies of MgB.sub.2, having a density close to the theorical value, which comprises the following passages: mechanical activation of crystalline boron with the formation of activated powders; formation of a porous preform of said powders; assembly of the porous boron preform and massive precursors of metallic magnesium in a container and sealing thereof in an atmosphere of inert gas or with a low oxygen content; thermal treatment of the boron and magnesium as assembled above, at a temperature higher than 700.degree. C. for a time greater than 30 minutes, with the consequent percolation of the magnesium, in liquid phase, through the activated crystalline boron powders.