We investigated the gas-surface interaction between oxygen molecules and ionomer surface using molecular dynamics simulations because the scattering phenomena of gas molecules on the surface play a crucial role for the gas transport in catalyst layers (CLs). The probability density functions (PDFs) of the translational energy of scattered oxygen molecules was analyzed for a wide range of incident condition and water contents for different scattering process. The energy distributions show the dependence on the incident temperature and independent of water content. The characteristic temperature of translational energy was also investigated by fitting to the Maxwell-Boltzmann distribution. This result suggests that there is a different tendency of the thermal accommodation to the ionomer surface depending on the scattering process.