Chemical decomposition of polymer electrolyte membrane (PEM) by hydroxyl radicals has been reported as one of causes of polymer electrolyte fuel cells (PEFCs) degradation, and as a countermeasure, a method of adding cerium ions into the PEM has been studied. Using molecular dynamics simulations, we constructed a system of the PEM that incorporate cerium ions to investigate the effect of cerium ions on the proton transport property. We have obtained proton diffusion coefficient, radial distribution function, coordinate number, number of cluster and cluster size. As a result, we found that at low water content, diffusion coefficient of proton increases by adding cerium ions because cerium ion attract the surrounding water molecules and clusters are connected to make better diffusion path of protons. At high water content. The presence of cerium ions in the cluster hinders proton transport, which results in the decrease in the diffusion coefficient of proton.