The adsorption of NO2 molecules on a series of zigzag (n,0) single-walled carbon nanotubes (SWNTs) (n = 6-12) have been investigated by first-principles methods. The results indicate that the tube diameter and the concentration of NO2 gas determine the interactions between NO2 molecules and the tube. The chemisorption of a single NO2 is only possible for the tube with a small diameter (n < 10), while the second NO2 molecule can be chemisorbed for all tubes studied here. The additions of more NO2 molecules in different patterns have also been considered for a (8,0) tube, and the NO2 groups prefer the pair arrangement with stronger binding energy. According to the results of band structure calculations, overall, the SWNT exhibits a p-type response upon exposure to NO2 gas and the electron charge transfer is an important reason for explaining the enhancement of conductivity of the tube. Moreover, it is interesting that, accompanied by charge transfer, the chemisorption of NO2 also leads to the radical transfer from the NO2 group to the carbon atoms at the ortho and para sites of the six-membered ring. As a result, the SWNT possesses the radical characteristic, which facilitates the further functionalization of the tube wall.