We fabricated well-defined Pt-Ir bimetallic surfaces through monolayer-thick Pt epitaxial growth on Ir(111) substrates (PtnML/Ir(111):n = 1-4) using molecular beam epitaxy in ultrahigh vacuum (UHV). The surface structures of PtnML/Ir(111) were verified using reflection high-energy electron diffraction in UHV. Electrochemical characterization tests, including evaluations of oxygen reduction reaction (ORR) activity and durability, were performed using cyclic voltammetry and rotating disk electrode techniques in 0.1 M HClO4. Regarding the initial ORR activity for PtnML/Ir(111) surfaces, Pt2ML/Ir(111) surfaces exhibited the highest ORR activity (24 times that of the clean Pt(111) surfaces). Initial ORR activities declined when square-wave potential cycles (PCs) were applied between 0.6 and 1.0 V vs. reversible hydrogen electrode. The Pt4ML/Ir(111) surface exhibited the highest ORR activity even after 5000 PC loadings (6 times that of the clean Pt(111) surfaces), suggesting that Pt-shell/Ir-core core-shell nanoparticles have potential for use as practical Pt-based bimetallic ORR catalysts.