The preparation of a Mg-Ni alloy (Mg100-xNix x=7-79) by mechanical alloying is studied. The results show that a great difference exists between the as-milled product and the conventional product. For x=7, 12, the as-milled products are mixtures of the intermetallic compound Mg2Ni and crystalline Mg. For x=18 and 25, the final products are the single phase nanocrystalline Mg2Ni. In the case of x=33, 43, 55, 67 and 70, the as-milled powders are the amorphous and the amorphization reaction paths are strongly dependent on the compositions. For x= 70, a single fee supersaturated solid solution forms. The mechanism of transformation is also discussed in detail.Correspondent: Ji Shijun, Lecturer, Institute of Materials and Technology, Dalian Maritime University, Dalian 116026, P. R. China,Tel:086-441-4671611