Molecular dynamics (MD) simulations were executed to clarify origins of the experimentally-observed high interfacial resistivity against the O-2 permeation through the Nafion ionomer thin film coated on the Pt surface, which is known to limit the power density of polymer electrolyte fuel cells with low Pt loadings. To describe the structures and transport properties of O-2 molecules inside the Nafion ionomer accurately, a new set of force field functions and parameters are proposed on the basis of the DFT calculations on molecular fragments composing the interfacial materials. MD simulations using the new parameter set reproduced well the experimentally-measured self-diffusion coefficient, solubility and permeation flux of O-2 molecules in the ionomer, and detailed free energy analysis indicated the presence of two distinctive barriers limiting the O-2 permeation flux at the gas/Nafion interface and Nafion/Pt interface. The former barrier dominates the O-2 solubility in the ionomer, and the latter barrier dominates the interfacial permeation resistivity at the Nafion/Pt interface. The rate-determining step of the O-2 permeation was judged as the interfacial permeation at the Nafion/Pt interface, and the high interfacial resistance at the Nafion/Pt interface was attributed to a dense ionomer-layer formed near the Pt surface. Suppressions of their formations were, therefore, considered to be significant for enhancing the O-2 permeation through the ionomer thin film. (C) 2015 Elsevier Ltd. All rights reserved.