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.