The increase in oxygen binding energy was previously proposed to account for the lower oxygen reduction activity of a Pt monolayer supported on Au(111) single crystal than that on Pd(111) and pure Pt(111) surfaces. This single-crystal based understanding, however, cannot explain the new finding of a 1.6-fold increase of oxygen reduction activity on Pt monolayer-modified 3-nm Au nanoparticles (Pt/Au/C) in comparison with that on Pt/Pd/C with a similar particle size. The Pt/Au/C catalyst also has an activity higher than that of a state-of-the-art 2.8-nm Pt/C catalyst. Our new experimental results and density functional theory calculations demonstrate that a significant compressive strain in the surface of the core nanoparticles plays a role in the observed activity enhancement.