We investigated the atomic-scale structure and electrochemical stability of a Pt-enriched topmost surface (Pt-enriched Ni/Pt(111)) prepared through monolayer Ni deposition on Pt(111) at 823 K using molecular beam epitaxy. Reflection high-energy electron diffraction patterns and an ultra-high vacuum scanning tunneling microscopic (UHV-STM) image of the Pt-enriched Ni/Pt(111) surface showed that the surface has long-range-ordered six-fold symmetry with atomic-scale corrugations. Although the oxygen reduction reaction (ORR) activity of the as-prepared Pt-enriched surface was eight times higher than that of clean Pt(111), the ORR activity degraded steeply during potential cycles between 0.6 and 1.0 V in O2-saturated 0.1 M HClO4. After 1000 potential cycles, the enhancement factor was estimated to be 2. An STM image collected after the potential cycles showed island-like structures (ca. 5-9 nm in diameter and 0.4-0.8 nm in height). Furthermore, the intensity of the Ni 2p core-level band for the as-prepared sample decreased after the 1000 potential cycles, revealing dissolution of the Ni atoms located at the subsurface layer. This experimental study clearly demonstrates that the underlying-Ni-induced specific surface strains and electronic state of the Pt-enriched topmost surface sorely contribute to the remarkable ORR activity of the Ni/Pt(111) surface.