The Mg-13Y bulk alloy was prepared by conventional casting process and the Mg-13Y powder was processed by ball milling using the casting alloy under the protection of argon. The hydrogenation thermodynamics, hydrogenation process and phase transitions were carefully investigated in the Mg-13Y powder alloy. It is shown that the Mg-13Y casting alloy consists of Mg24Y5 phase and alpha-Mg containing yttrium which have different hydrogenation enthalpies, -195 kJ/mol H-2 (by calculation) and -42 kJ/mol H-2 (by Pressure-Composition-Temperature experiment), respectively. The structure evolution and phase transition in the Mg-13Y bulk alloy treated at 673 K and at 4 MPa for 40 h were observed by an optical microscopy (OM), a scanning electron microscopy (SEM), a transmission electron microscopy (TEM) and X-ray diffraction (XRD). The large Mg24Y5 phase in the bulk Mg-13Y alloy could be destroyed into fine cuboid-shaped YH2 phases during the hydrogenation process, which is probably responsible for the improvement of mechanical properties of Mg-13Y alloy. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.