A DFT study is performed to understand the underlying mechanisms behind the improvement of hydrogen storage properties of MgH2 doped with different transition metals TM = Sc, Ti, V, Cr, Y, Zr, Nb, Mo. These TMs are either from the same period or the same group of the periodic table. Our calculations are performed using all-electron full potential local-orbital minimum-basis scheme (FPL09.00-34) within the generalized gradient approximation. Based on our results, we observe a reduction of the heat of formation OH and also of the desorption temperature T-d which reflects the improvement of hydrogen storage properties of most TMs doped MgH2 systems. In particular, Nb appears to be the best substitution, with an ideal heat of formation that equals to -40.96 kJ/mol, and a desorption temperature T-d = 313.40 K. Our findings demonstrated that, the decrease of Delta H and T-d is, mainly, related to the weakening of the bond between Mg and H. A detailed study of the effect of each TM on the stability of MgH2 was carried on. Our discussion was based on the charge transfer between the system elements, the difference of electronegativity, and finally the density of states. Copyright (c) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.