Abstract Using first-principles calculations based on density-functional theory, we systematically investigate the influence of adsorbates (CO molecule and O atom) and defects (adsorb one extra Cu atom and monovacancy) on the structural and electronic properties of Cu 5-1 NW and Cu 6-1 NW. For both nanowires, CO molecule prefers to adsorb on the top site, while O atom prefers to adsorb on the center site. The hybridization between the CO and Cu states is dominated by the donation–backdonation process, which leads to the formation of bonding/antibonding pairs, 5σ b /5σ a and 2π b ⁎ /2π a ⁎ . The larger adsorption energies, larger charge transfers to O adatom and larger decrease in quantum conductance 3 G 0 for an O atom adsorbed on the Cu 5-1 NW and Cu 6-1 NW show both Cu 5-1 NW and Cu 6-1 NW can be used as an O sensor. Furthermore, the decrease in quantum conductance 1 G 0 for a CO molecule adsorbed on the Cu 6-1 NW also shows the Cu 6-1 NW can be used to detect CO molecule. So we expect these results may have implications for CuNW based chemical sensing. High adsorption energy of one extra Cu atom and relatively low formation energy of a monovacancy suggest that these two types of defects are likely to occur in the fabrication of CuNWs. One extra Cu atom does not decrease the quantum conductance, while a Cu monovacancy leads to a drop of the quantum conductance.