The diffusion process of O-2 in the cation-form Nafion membrane is investigated by molecular dynamics simulations. Moreover, the effects of cation species and concentration on O-2 diffusion are elucidated systematically. The results show that for the H+-form Nafion membrane, the average size of hydrophilic clusters in the membrane increases and these clusters will interconnect gradually as the membrane water content increases. Thus, the diffusion coefficient of O-2 increases with the increase of water content. In addition, O-2 molecules tend to diffuse along the hydrophilic/hydrophobic phase interface and within the hydrophobic phase of the membrane. Na+ or Ca2+ promotes while K hinders O-2 diffusion compared with H+ when there is only one kind of cation in the Nafion membrane. As to the membrane contains both H+ and one kind of metal cations mentioned above, although the ratios of these two kinds of cations change linearly, the diffusion coefficient of O-2 changes nonlinearly. It is mainly because that the factors affecting O-2 diffusion in the Nafion membrane have different priorities under different cation species and concentration conditions. In general, the factors such as the large free volume fraction of the membrane, the large average size of hydrophilic clusters with better connectivity and the wide distribution of O-2 molecules close to the hydrophobic phase are conducive to O-2 diffusion in the cation-form Nafion membrane. (C) 2018 Elsevier Ltd. All rights reserved.