Density functional theory calculations are performed to study the healing mechanism of single-vacancy defects in zigzag (n,0) CNTs by NO molecule (n = 6,8,10). The results indicate that the healing process proceeds through a two-step mechanism. First, NO molecule adsorbs over the defective site. Then, the extra oxygen atom (O-ads) is eliminated by three different ways: (i) NO + O-ads -> NO2, (ii) CO + O-ads -> CO2, or (iii) SO2 + O-ads -> SO3. The dependency of the healing process on the tube diameter is studied in detail. The results of this work suggest a novel approach to achieve N-doped CNTs. (C) 2017 Elsevier B.V. All rights reserved.