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Copper is a potential electrocatalyst for CO2 electroreduction, although controlling the selectivity towards hydrocarbons and CO is still a major challenge. It is known that Cu nanoparticles show a better performance for CO2 electro‐reduction than bulk Cu. However, the roles of the size and symmetry of the Cu clusters as well as the temperature in the CO2‐reduction process remain elusive, which hinders the development of advanced catalysts. In this study, the density functional theory (DFT) method is applied to investigate these factors. We find that the decrease of icosahedron Cu clusters' size but the increase of truncated octahedron Cu clusters' size contributes to the selectivity of CO2 reduction. In addition, the symmetry of Cu clusters modulates the selectivity of CO2 reduction at room temperature: the (111)‐like surfaces prefer to produce hydrocarbons but the synergistic effect between (100)‐ and (111)‐ like ones favors the formation of CO, which are in reasonable agreement with the experimental measurements that distinct products are detected on different Cu nanoparticles during CO2 reduction. We also note that the increase of temperature is favorable to CO production. Our findings not only contribute to a thorough understanding of CO2 reduction on Cu clusters, but also provide clues for designing catalysts in future experiments. CO2 reduction on Cu clusters strongly depends on the cluster size and reaction temperature. Distinct products are obtained on clusters with different symmetries at room temperature. It is also found that high temperatures can facilitate CO desorption. [ABSTRACT FROM AUTHOR]