We investigated surface structures and oxygen-reduction reactions (ORR) of Pt-Pd(111) model bimetallic surfaces fabricated by molecular beam epitaxy. The Pt/Pd(111) surfaces were prepared by depositing 0.6-nm-thick Pt on Pd(111) at substrate temperatures of 573 and 673 (xK-Pt0.6nm/Pd(111), where x = 573 or 673). Reflection high-energy electron diffraction patterns and ultra-highvacuum scanning tunneling microscopy images revealed that Pt grew epitaxially on the Pd(111) substrate under the deposition conditions selected for the study. However, analysis of low-energy ion scattering results suggested that the Pt and Pd atomic compositions of the topmost surfaces were different for 573KPt0.6nm/ Pd(111) and 673K-Pt0.6nm/Pd(111). The ORR activities of the 573K-Pt0.6nm/Pd(111) and 673K-Pt0.6nm/Pd(111) surfaces were 6.3 and 3.6 higher than that for Pt(111), respectively. The results indicate that alloy structures near the surface determine the ORR activity of practical Pd@Pt core-shell catalysts.