Deposition of platinum (Pt) monolayers (PtML) on Au substrate represents a robust strategy to maximally utilize the Pt atoms and meanwhile achieve high catalytic activity towards methanol oxidation reaction for direct methanol fuel cells owing to a substrate-induced tensile strain effect. However, recent studies showed that PtML on Au substrate are far from perfect smooth monoatomic layer, but actually exhibited three-dimensional nanoclusters. Moreover, the PtML suffered from severe structural instability and thus activity degradation during long-term electrocatalysis. To regulate the growth of PtML on Au surface and also to improve its structural stability, we exploit dealloyed AuCu core-shell nanoparticles as a new substrate for depositing PtML. By using high-resolution scanning transmission electron microscopy and energy dispersive X-ray elemental mapping combined with electrochemical characterizations, we reveal that the dealloyed AuCu core-shell nanoparticles can effectively promote the deposition of PtML closer to a smooth monolayer structure, thus leading to a higher utilization efficiency of Pt and higher intrinsic activity towards methanol oxidation compared to those on pure Au nanoparticles. Moreover, the PtML deposited on the AuCu core-shell NPs showed substantially enhanced stability compared to those on pure Au NPs during long-term electrocatalysis over several hours, during which segregation of Cu to the Au/Pt interface was revealed and suggested to play an important role in stabilizing the PtML catalysts.