The synthesis process of Mg–Ti alloys with a BCC (body centered cubic) structure by means of ball milling was studied by X-ray diffraction and various microscopic techniques. The morphology and crystal structure of Mg–Ti alloys changed with increase of milling time. During ball milling of Mg and Ti powders in molar ratio of 1:1, firstly, plate-like particles stuck on the surface of the milling pot and balls. After these plate-like particles fell off from the surface of the milling pot and balls, spherical particles with the mean diameter of 1mm, in which concentric layers of Mg and Ti were disposed, were formed. These spherical particles were crushed into spherical particles with the diameter of around 10μm by introduction of cracks along the boundaries between Mg and Ti layers. Finally, the Mg 50 Ti 50 BCC phase with the lattice parameter of a=0.342(1)nm and the grain size of 3nm was formed. During milling of Mg and Ti to synthesize the BCC alloy, Mg and Ti were deformed mainly by the basal plane slip and the twinning deformation, respectively. Ti acted as abrasives for Mg which had stuck on the surface of the milling pot and balls. The BCC phase was found after Mg dissolved in Ti.