Detailed density functional theory (DFT) calculations have been carried out to study the ammonia (NH3) decomposition activity and catalytic mechanism for octahedral nickel nanoclusters. Four different size nickel nanoclusters, Ni-19, N-144, Ni-85 and Ni-146, were applied to investigate the nanoscale size effect. The results reveal that these four nickel nano clusters adopt the same NH3 decomposition mechanism. The predicted catalytic activities of these nickel nanoclusters decrease in the sequence of Ni-44 > Ni-146 approximate to Ni-85 > Ni-19, based on the analysis of adsorption energies of NH3 decomposition intermediates and landscapes of NH3 decomposition pathways. Besides, the reaction heat of Ni-146 nanocluster is only 0.65 eV in the process of N-2 desorption, showing the nature of easy desorption out of the catalyst surface. Moreover, molecular dynamics calculations show that large-size nano clusters performed better on thermal stability. Combined with dynamics analysis, we found that Ni-146 nanocluster possesses not only high thermodynamic stability but also catalytic activity. Electronic structural analysis show that negatively Mulliken charged sites were more favorable for adsorbing N and act as active centers. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.