We demonstrated the preparation of Mg17Al12, a promising hydrogen storage alloy, by the method of combustion synthesis, and focused on the effects of the aluminum particle size, the synthesis temperature and the corresponding holding time on the phase composition of the products. From the X-ray diffraction patterns, it was confirmed that both the decrease of the aluminum particle size and the increase of the holding time kept at the synthesis temperature were beneficial to improve the purity of the products dramatically. It is notable that Mg17Al12 single phase can be obtained with only 0.5 h holding time at 733 K when the aluminum particle size is 1-3 mu m. What is more, we first proposed the alloying mechanism in the combustion synthesis of Mg-Al alloy, that is, Mg2Al3 phase is synthesized first, then Mg2Al3 reacts with residual Mg, forming Mg17Al12 alloy. (C) 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.