Abstract: The present invention relates to a method for producing grain refined Mg and Mg alloys by solidifying molten Mg or Mg-alloys in the presence of ZrB2 or ZrC particles leading to a grain refined microstructure. The particles have an average particles size less than 20 ?m and larger 0.1 ?m and are either produced by an in situ formation within the melt or by a high temperature treatment of an intimate mixtures of zirconium oxide and carbon black and in case of ZrB2 additionally a suitable boron source like boron carbide or boron oxide or boron followed by a milling and de-agglomeration procedure. The method according to the invention leads to grain refined Mg or Mg alloys with an average grain size less than 100 ?m.