An experimental investigation of the rate limiting steps for the desorption of magnesium hydrides is performed in this work to clarify the factors and corresponding mechanism determining the dehydrogenation process of MgH2. The effects of Mg nucleation and hydrogen atoms recombination on the desorption performance of MgH2 are investigated by ball milling MgH2 with Mg powders and Ni powders. The comparison between the effects of nucleate precursors and hydrogen atoms recombination is carried out by control experiment with metallographic observation and kinetic data analysis. The results show that the addition of Mg is mixed with MgH2 at the atomic scale and favors the nucleation of metals during dehydrogenation of MgH2, e.g., the activation energy is decreased by 10 kJ/mol with the addition of 10wt.%Mg. Ni improves the desorption kinetics to a great extent by accelerating the recombination of hydrogen atoms. The recombination of hydrogen atoms on the surface of newly formed metals limits the desorption process more considerably than the nucleation of Mg. The dehydrogenation kinetics of MgH2 can be significantly improved with the addition of nucleate precursors Mg and catalyst Ni powders. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.