The adsorption of CO molecules on Fe-doped single-vacancy-defected carbon nanotubes (Fe-SV-CNTs) was computationally investigated. Results show that the interaction of Fe-SV-CNTs with CO, although is slightly weaker than that of Fe-doped intrinsic CNTs, is much stronger than that of intrinsic CNTs and single-vacancy-defected CNTs (SV-CNTs). After CO adsorption, the electrical conductivity of Fe-SV-CNTs has a much more significant change than that of other types of CNTs. Considering that doping transition metals on the defects of SV-CNTs could be achieved much more easily than on the lattices of intrinsic CNTs, Fe-SV-CNTs would be more practical as CO gas sensing material.