Stoichiometric mixture Mg-Ni was prepared from Mg chips and Ni powder and milled under inert and hydrogen atmospheres for 22h using a centrifugal mill with ball to powder ratio 30:1. Hydrogen pressures used were 0.14, 0.28 and 0.42 MPa. The as-milled material was analyzed by SEM, TEM and XRD. Hydrogen contents of samples were determined using a LECO RH-402 analyzer. Remarkable differences on morphology and phases formation were observed for the two atmospheres we have used. Argon atmosphere didn't promote alloying while hydrogen atmosphere promoted formation of nanocrystalline Ni and Mg-rich amorphous phase. As hydrogen pressure increased more amorphous phase was formed.