Single-atom catalysts have attracted wide attention recently because of their unique size quantum effect and superior atom utilization in CO2 reduction reaction (CO2RR). Here, ultrathin Ni and nitrogen-codoped carbon nanosheets (Ni-N-CNSs) were proposed by a facile in-situ pyrolytic strategy. The ultrathin porous structure of Ni-N-CNSs affords large surface area, rich mesoporous volume and vast uniformly dispersed Ni atoms. The optimized catalyst exhibits a high CO Faradaic efficiency of nearly 100%, partial current density of 121.4 mA mg(-1), CO production rate of 37.7 mu mol mg(-1) min(-1), and super durability. In addition, the first principles calculations and the Mulliken charge analyses reveal the Ni sites show high bonding force towards the CO2 molecules, which gives rise to the high activity and selectivity of Ni-N-CNSs in CO2RR. (C) 2020 Elsevier Inc. All rights reserved.