A simple, new method utilizing the hydrolysis of Mg2Ni has been successfully developed for the synthesis of Ni nanoparticles. The possible growth mechanisms of Ni nanoparticles are discussed. Compared with conventional preparation methods for Ni nanoparticles, this method has the potential to inexpensively produce Ni nanoparticles on a large scale. In addition, the principle of the method could be applied to the synthesis of other transition metal nanoparticles such as Co, Cu, Ag, Au, Pt, and Pd.