Abstract: A typical highly effective magnesium-containing catalyst prepared by chemical reactions was studied. Each reaction step in the preparation was examined by X-ray powder diffraction, IRand chemical analysis.In the first step, C₂H₆OHreacts with MgCl₂ in gasoline, the lattice of MgCl₂ crystal is broken and a solid adduct, MgCl₃·nC₂H₅OH (n≤6), is formed. In the second step, by adding Al(iso-C₄H₉)₃, MgCl₃ · nC₃H₆OHdisappears and a complex solid product containing Mg⁺², Al⁺³, Cl^- and OC₂H₅^- is formed. Finally TiCl₄ is added, a powder product as the main component of a highly effective catalyst for ethylene polymerzation is obtained. In this last step, most of Al⁺³ and OC₂H₅^- in the solid product from the second step dissolves in the liquid medium, thus Mg⁺³ and Cl^- ions recombine to form MgCl₂ crystallites as a supporter, which can be identified by X-ray diffraction. The compound of titanium, the active ingredient of the catalyst, is highly dispersed on the surface of MgCl₂ crystallites and thus a highly active catalyst is obtained which contains plenty of interstices between the crystallites, and the transfer rate of ethylene during polymerization is enhanced. Hence the activity is higher than the activity of those produced by the milling method.MgCl₂ crystallites are found in some other Mg-containing catalysts prepared by chemical reactions with different materials, so that they should have a similar mechanism of high activity.