To explore the effect of strong metal/support interaction (SMSI) on the activity and stability of Pt/CaMnO3 catalyst for oxygen reduction reaction (ORR), we investigate the geometry and electronic structure of CaMnO3-supported Pt-5 cluster system, as well as its reactive properties toward ORR using density functional theory. The results show that a moderate amount of O-vacancies obviously improve the electrical conductivity of CaMnO3 through narrowing its energy gap, which induces a lower electron transfer resistance of CaMnO3 particles themselves. When the Pt-5 cluster is loaded on CaMnO3, a stronger interaction will form between the Pt-5 cluster and CaMnO3 than that between Pt-5 cluster and graphene, since more charge transfer from support to CaMnO3, and larger overlap between O-p states of CaMnO3 and Pt-d states of Pt-5 cluster In addition, for Pt-5/CaMnO3, it can be found that charge transfer from catalyst to O-2 promotes dissociation of O-2, and lowering of d-band center of Pt-5 facilitates desorption of intermediates when compared with Pt-5/graphene. (C) 2013 Elsevier Ltd. All rights reserved.