In this work, initial activation mechanism of CO2 over MgO supported Ni catalysts has been systematically studied through the periodic DFT calculations. In addition, the role of metal cluster, interface and support for CO2 activation is investigated and the active site is identified. CO2 is most favored to be activated on the interface instead of neither Ni cluster nor MgO support. The effective energy for this process is around 0.67 eV, and the dissociation of CO2 (0.62 eV) is the rate-determining step, since it requires much higher energy than that of the CO2 adsorption process (0.05 eV). Thus, the interface between metal cluster and support plays a key role for C=O bond activation. Moreover, CO* is preferred to be adsorbed on the Ni cluster, while the O* is likely to bind on Mg atom of support. To illustrate the adsorption behavior of CO2 at different sites, the Mulliken atomic charge and electron density difference have been calculated. It was found that the total amount of electron gain for CO2 binding at different sites follows the order of Interface (-0.03 e) < MgO support (-0.05 e) < Ni cluster (-0.07 e), and effective energy barrier rises linearly with the increase of electron gain of CO2 binding at different sites. In addition, electron gain of oxygen atom O1 and oxygen atom O2 of CO(2 )is the same for Ni cluster and MgO support, however, the electron gain of 01 and 02 is different for Interface. The difference of electron gain for two oxygen atoms shows the electron unbalance of CO2 molecule, which is in favor of C=O activation. This study could shed some light on understanding the active sites of CO2 thermal-catalytic activation over MgO supported Ni catalysts, and is helpful to elucidate the reaction on an atomic level. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.