Abstract: Adsorption of NH₃ molecules on the carbon nanotubes(CNTs),including the CNTs with and without a single-vacancy defect and/or doped Mn-atom,was investigated in the first-principles method based on density functional theory.The adsorption energy,adsorption distance,charge-transfer and electron density were analyzed.The calculated results show that the single-vacancy defect and Mn-doping has a major impact.For instance,a single-vacancy defect slightly increases the adsorption of NH₃,though fails to induce chemical adsorption with CNTs;the doped Mn-atom significantly shortens the adsorption distance,considerably increases the adsorption energy and charge transfer,and possibly promotes a stable chemical adsorption between NH₃ and CNTs with a single-vacancy defect.We suggest that the Mn-doped CNTs with a single-vacancy defect may be of some basic and technological interest in synthesis of advanced NH₃-gas sensitive material.