The electrochemical CO2reduction reaction (CO2RR) which produces high value-added products is regarded as a prospective path toward carbon cycling. However, the challenges in the electrochemical CO2RR are its competition to the hydrogen evolution reaction (HER) and unsatisfied product selectivity. Therefore, the design and preparation of highly effective catalysts with low overpotential, high faradaic efficiency (FE), and high selectivity are key considerations for the development of the CO2RR technology. In this research, binary In-Sn catalysts with In/Sn atomic ratios of 3/1 and 1/1 have been prepared. In-Sn nanoparticles (NPs) are composed of In and InSn4with some surface In and Sn oxides. Sn addition enhances the partial current density of formate production during the CO2RR. In3Sn/C NPs show outstanding performance with a HCOOH FE of 92.6% and excellent durability for 10 h at -1.0 V. In addition, the comparison of the binary In3Sn/C and physical mixture (In/C and SnO2/C) highlights that In4Sn is responsible for the high activity and stability of formate conversion. This research provides guidelines for the design of CO2RR catalysts with high formate conversion by optimization of compositions and structures of InSn NPs.