The composition, phase components, and microstructure of Mg -based Sm5Mg41 alloy prepared by vacuum induction melting technique were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and high resolution transmission electron microscopy (HRTEM). The gaseous hydrogen ab/desorption properties of the alloy were measured by an automatically controlled Sieverts apparatus. The results indicate that the as-cast alloy consists of two phases, major phase Sm5Mg41 and secondary phase SmMg3. The MgH2 and Sm3H7 phases form after hydrogen absorption, while Mg and Sm3H7 phases exist after hydrogen desorption at 340 degrees C. Field Emission Transmission Electron Microscopy (FETEM) observation reveals the microstructure and phase distribution of Mg-based Sm5Mg41 alloy before and after hydrogen absorption and the hydriding and dehydriding reaction pathways as follow: Sm5Mg41 + SmMg3 + H-2 -> Sm3H7 + MgH2 <-> HSm3H7 + Mg + H-2. The hydrogen storage thermodynamics and kinetics of the Mg -based Sm5Mg41 alloy are improved as a result of the formation of the Sm3H7 nanoparticles. Consequently, the starting dehydrogenation temperature of the alloy hydride is about 270 degrees C. The dehydrogenation and hydrogenation activation energies of the alloy are estimated to be 135.28 and 77.802 kJ/mol, which suggests that the Sm3H7 nanoparticles play a beneficial role to reduce the total potential barrier that the hydrogen absorption or desorption reaction must overcome. The hydrogenation enthalpy of the alloy was determined to be -76.52 kJ/mol H-2, indicating that adding Sm can slightly alter the hydrogen absorption thermodynamic property of Mg -based alloy. The desorption property improved by alloying Sm is attribute to the enhanced kinetics rather than the variation in the thermodynamics. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.