The effect of Ni-Mg particle size on product selectivity, catalyst activity and coke formation on Ni/MgO/alpha-Al2O3 catalyst were studied in simultaneous steam and CO2 reforming of methane to syngas. The Ni-Mg particle sizes in the range of 13.7-29.7 nm were prepared via incipient wetness impregnation method using alpha-Al2O3 cc as support. Experimental results for turn over frequency (TOP) show that the intrinsic catalyst performances were dependent on Ni-Mg particle size. However, the products selectivities and H-2/CO ration were found to be independent of Ni Mg particle size. The experimental results reveal that the coke formation rates on the surface of catalysts are increased by increasing Ni Mg particle size. Nevertheless, the correlated site carbon atom deposition rates (carbon atoms deposited on one site of catalyst) is shown an opposite trend. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.