Although a copper catalyst has very interesting properties in CO2 electroreduction reaction (CO2RR), the high overpotential of this reaction and low selectivity of the catalyst for a single product are major hindrances to catalyst commercialization. In this work, monodisperse Cu-Pd nanoparticles (NPs) with various compositions are synthesized using the colloidal method. These NPs show a totally different catalytic performance than bulk Cu catalysts. Alloying Cu with Pd suppresses hydrocarbon production on the alloy NP catalyst surface. NPs with a 1:1 Cu -Pd ratio show the best catalytic activity for the conversion of CO2 to CO. At -0.9 V (vs. RHE), 87% CO Faradaic efficiency is achieved, as well as a high noble metal mass activity of 47 mA mg(Pd)(-1), for CO production. Density functional theory calculations suggest that the energy barrier to the CO* protonation step is increased when Pd is alloyed with Cu; this increase suppresses the reduction of CO2 to hydrocarbons. This result is a significant advance toward selective electrochemical reduction of CO2.