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Binding energies and geometrical and electronic structures for adsorptions of CO and NO on metal M (Pd or Pt) loaded or M and di-vacancy co-decorated (5,5) single-walled carbon nanotubes (M-CNTs or M-V2-CNTs) are studied using a GGA-PBE method in the work.The calculated results show that the di-vacancy defect in a perfect (5,5) tube opens the band gap,makes the (5,5) tube transform from a conductor into a semiconductor,and strengthens the adsorption of metal M on the (5,5) tube.For the adsorptions of CO and NO on M-CNT and M-V2-CNT,the CO and NO molecules can be both chemically adsorbed on loaded Pd or Pt atoms due to their active adsorption sites.NO is easily adsorbed on M-V2-CNT because of its electron configuration with a high 2π energy level and its adsorption significantly changes the band gap of M-V2-CNT and makes M-V2-CNT transform from a semiconductor to a conductor.However,the adsorption of CO can not cause the conductivity of M-V2-CNT change.M-V2-CNT has a good sensitivity to the NO gas,suitable as a sensor for detecting the NO gas molecule.In addition,the existence of di-vacancy defect decreases the interaction between CO or NO and Pt-CNT,which will contribute to the desorption of CO and NO gases.The work is expected to provide a theoretical basis for designing NO sensing devices.