Metal oxides have been widely studied as promoter of catalytic performance of metal catalysts through the strong metal-support interactions. However, the mechanism behind it is still debating. Here a series of PdCu nanoparticles with size from 4.9 to 15.4 nm are prepared and loaded on CeO2 nanorods, which are further characterized by HRTEM, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical CO stripping. The coupling of PdCu nanoparticles and CeO2 nanorods induces compressive strain on PdCu nanoparticles, which is strongly dependent on the size of PdCu nanoparticles. The correlation among the compressive strain, down-shift of the d-band center, reduced CO binding energy and augmentation of PdO content for PdCu nanoparticles is further discussed in details. Based on comprehensive analyses, it is concluded that the reduced CO oxidation potential on PdCu nanoparticles upon CeO2 support is from enhanced PdO on PdCu, while the synergy of compressive strain, enhanced PdO and oxygen vacancy remarkably promotes the catalytic performance of PdCu towards electrooxidation of formic acid. (C) 2019 Elsevier Inc. All rights reserved.