The in situ X-ray diffraction technique is adopted to determine the degree of reaction between Mg and B at several isothermal temperatures below Mg melting. Based on these isothermal data, a kinetics analysis of MgB2 phase formation during the low-temperature sintering is carried out. It is found that the reaction between Mg and B is controlled by varied mechanisms. In the initial stage, the reaction rate is mainly controlled by a phase boundary reaction mechanism. As the reaction proceeds, the diffusion-limited mechanism gradually becomes dominant. The corresponding activation energy also firstly decreases, and then increases again during the whole reaction process. Combined with the classic solid-sintering theory, the determined kinetics model is valid and consistent with the actual sintering process. (C) 2010 Elsevier B.V. All rights reserved.