We investigated the electrochemical properties of various-monolayer (ML)-thick Pt-shell model surfaces. The m-ML-Pt shell/Au(111) (m = 1-3) was prepared through epitaxial growth of Pt on a clean Au(111) surface at 300 K, and the n-ML-Pt shell/Ni-Pt(111) (n = 1-4) was generated through thermal segregation of substrate Pt atoms during 1-ML-thick Ni deposition onto a clean Pt(111) surface at 823 K followed by additional epitaxial growth of Pt at 473 K in ultra-high-vacuum molecular beam epitaxy (UHVMBE) systems. Surface structures of the Pt-shell surfaces were verified using a scanning tunneling microscope in UHV. Oxygen reduction reaction (ORR) activities and structural stabilities of the shell surfaces were evaluated in oxygen-saturated 0.1 M HClO4. The results obtained reveal that not only surface strains but also nanoscale surface morphologies of the most dense plane of Pt(111) are correlated with ORR enhancements and that the 3-ML-thick Pt shell stabilizes the topmost surfaces of Pt/M bimetallic systems.