We present the systematic trends in the catalytic activities of 3d transition metals M (M: Sc to Zn) on the Mg-H dissociation of magnesium hydride MgH2 using ab initio calculations based on density functional theory (DFT). In order to represent the MgH2 + M systems, we put the M atom on an MgH2 cluster as a model for the calculations. We calculated the energies required for increasing the Mg-H distance in the presence of the M. As a result, we found that Sc and Ni have higher catalytic activities than the other elements, resulting from their 3d orbital occupancies. (c) 2005 Elsevier B.V. All rights reserved.