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Size effect has been regularly utilized to tune the catalytic activity and selectivity of metal nanoparticles (NPs).It is still a computational challenge to directly simulate the CO2 reduction over Pd NPs with different sizes by means of DFT calculations.However, it is possible to simulate the process over three different reaction sites (terrace, edge, and comer) of Pd NPs, represented by flat Pd(111) surfaces, stepped Pd(211) surfaces, and Pd55 (or Pd38) clusters, respectively.Furthermore,according to the ratio of three kinds of reaction sites (terrace, edge, and comer) on ideal Pd NPs with different sizes, it is reasonable to build the intrinsic relationship between catalytic properties and NPs size[1].We report a computational understanding of the size effect in the electrocatalytic reduction of CO2 over differently sized Pd NPs, ranging from 2.4 to 10.3 nm.DFT calculations found that comer and edge sites on Pd NPs were more active than terrace sites toward CO2 reduction and that competitive HER was similar on all three sites.The size dependence of the turnover frequency for CO production suggests that CO2 adsorption, COOH* formation and CO* removal during CO2 reduction can be tuned on differently-sized Pd NPs due to the change in the ratio of comer, edge and terrace sites.