The support effect and metal particle size effect, which are often intertwined, are two critical factors influencing the activity of a supported metal catalyst. To clarify each effect on the activity of carbon supported Pt catalysts, a series of Pt/C catalysts were elaborately prepared by adopting two carbon materials with distinct microstructures-that is, carbon nanofibers (CNFs) vs carbon nanotubes (CNTs)-and by altering the Pt particle size from 1 to 9 nm on each support. The catalytic activity was assessed during the dehydrogenation of decalin to release hydrogen. Molecular dynamics simulations based on reactive force field and density functional theoretical calculations were employed in combination with catalyst characterization techniques. Significant differences in both the geometric structure and electronic properties of Pt clusters between two kinds of Pt/C catalysts have been found, especially in the Pt particle size range smaller than 2 nm. The edge planes of CNFs can interact much more strongly with Pt atoms than the basal planes of CNTs do, hence forming a more dispersive and simultaneously more active Pt-C boundary for reaction proceeding. The carbon support effect dominates the activity divergence in the smaller size range (<2 nm), while the Pt particle size effect prevails on the catalysts with medium Pt particles (2-4 nm). Both effects can fade out with continually increasing Pt particle size. (C) 2018 Elsevier Inc. All rights reserved.