A series of carbon aerogel (CA) samples with different pore sizes were loaded with platinum nanoparticlesthrough the polyol reduction method. All the Pt/CAs possess nano Pt particles (2.5−3.5 nm) and uniformdispersion. The catalytic activities to oxygen reduction reaction (ORR) of Pt/CAs were evaluated by cyclicvoltammetry in O2 saturated HClO4 with a rotating disk electrode. The catalytic activities of Pt/CAs arehigher than that of commercial carbon black supported catalyst with the same Pt loading amount. Among thePt/CAs, the relationship between ORR activity and mean meso pore size shows that the greater meso poresize can promote the catalysis activity. In the present study, the carbon aerogel with the mean meso pore sizeof 18.5 nm achieves the best ORR performance.