Ultrasmall gold (Au) nanoparticles with high mass activity have great potential for practical applications in CO2electroreduction. However, these nanoparticles often suffer from poor product selectivity since their abundant low-coordinated sites are favorable for H2evolution. In this work, a catalyst, reduced graphene oxide supported ultrasmall Au nanoparticles (≈2.4 nm) is developed which delivers high Au-specific mass activities (>100 A g-1) and good Faradaic efficiencies (32-60%) for the CO2-to-CO conversion at moderate overpotentials (450-600 mV). The efficiencies can be improved to 59-75% while retaining the ultrahigh mass activities via a simple amine-modification strategy. In addition, an amine-structure-dependent effect is revealed: linear amines promote the CO formation whereas the branched polyamine greatly depresses it; the increasing alkyl chain length boosts the promotion effect of linear amines. The strong Au-amine interaction and molecular configuration induced amine coverage on the ultrasmall Au NPs may contribute to this effect.