© The Electrochemical Society We analyzed the transport properties of cerium ions, which are added to improve the chemical durability of a polymer electrolyte membrane (PEM), using molecular dynamics simulations. A system of the PEM in which incorporate cerium ions are incorporated and water flow is set was constructed to investigate the convective ion transport properties. At a low water contents, cerium ions are hardly affected by water flow. This is because many cerium ions are trapped by sulfonate group of Nafion polymer and the connectivity of water cluster is low. As the water contents increase, free cerium ions without being trapped by sulfonate groups increase and water clusters are connected and the cluster size increases. Hence, cerium ions are easily affected by water convection at high water contents.