Abstract Magnesium borohydride (Mg(BH 4 ) 2 ) is a promising material for hydrogen storage because of its high gravimetric storage density (15.0 mass%). We intended to synthesize Mg(BH 4 ) 2 by decomposition reaction of LiBH 4 with MgCl 2 by heat treatment without using a solvent, where the product consists of LiCl and a compound of magnesium, boron and hydrogen. Hydrogen desorption temperature of the product is approximately 100 K lower than that of LiBH 4 and the decomposition consists of a two-step reaction. The products of the 1st and 2nd decomposition reactions are MgH 2 and Mg, respectively. This result indicates the following two-step reaction (1st reaction: Mg(BH 4 ) 2 →MgH 2 +2B+3H 2 , 2nd reaction: MgH 2 →Mg+H 2 ). The first decomposition peak is dominant and is around 563 K. The 2nd decomposition occurs at the temperature greater than 590 K.