We report here the study on the kinetics and growth of Mg2Ni alloy layers by solid-state reactions in the Mg-Ni system. After appropriate heat treatments, only polycrystalline Mg2Ni phase was found to form on the Mg/Ni interfaces and the grain sizes of Mg2Ni formed near the Mg side were found to be much smaller than those formed near the Ni side. Cross-sectional SEM examinations revealed that the Mg2Ni intermetallic layers grew following the diffusion-control process. During the formation of Mg2Ni layers, the dominant diffusing species was determined to be Ni by using the immobile trench markers. Furthermore, based on the marker analysis, the intrinsic and interdiffusion diffusivities for the Mg-Ni diffusion couples at different temperatures have been determined.