In this paper, reaction kinetics in the combustion synthesis of Mg2Ni was discussed, in which physical and chemical changes of the samples during combustion synthesis were observed and identified by Scanning Electron Microscope (SEM), Electron Probe X-ray Microanalyzer (EPMA) and X-ray diffraction, and a rate equation of solid phase reaction was derived, based on the data measured by Differential Scanning Calorimeter (DSC). The SEM observation demonstrated that the synthesis reaction of Mg2Ni proceeds uniformly rather than topochemically, and an intermediate phase dose not exist during combustion synthesis. The synthesis reaction was accelerated by liquid generation of eutectic-composition. As a result, the rate equation of the sample was expressed by the form of a second-order irreversible equation, k(1−f)2, with the change of the frequency factor, and then it simulated reasonably the DSC curves of combustion synthesis.