The slow hydrogenation kinetics and high reaction temperature of Mg primarily limit its application for mobile hydrogen storage. H-2 adsorption and dissociation on the pure and TM-doped (TM = Ti, Nb) Mg-55 nanoclusters are systematically studied by using density functional theory (DFT) calculations. It is found that the introduction of Ti and Nb atoms into Mg-55 nanocluster can greatly modify the electronic structure of Mg-55 nanocluster and enhance the stability of system. Through the analyses of results from the climbing image nudged elastic band (CI-NEB) and reaction rate constant, we also find that the energy barriers of H-2 dissociation on TM-doped Mg-55 nanoclusters can be significantly decreased due to the addition of Ti and Nb. Adding Ti and Nb atoms can dramatically improve the rate constant of H-2 dissociation, especially for H-2 dissociation on Mg(54)TM2 (TM atom replacing the inner shell position), Mg(54)TM3 (TM atom replacing the outermost vertex) and Mg-54-TM4 (TM atom replacing the outermost edge position) nanoclusters. Moreover, compared with the Ti dopant, the Nb will generate a lower activation barrier for H-2 dissociation on TM-doped Mg-55 nanoclusters. We also suggest that the subsurface and surface positions (Mg(54)TM2, Mg(54)TM3, Mg(54)TM4) are the ideal substitutional sites for TMs. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.