An　MWCNT-doped　(multi-walled　carbon　nanotube)　SnO2　thin　film　NO2　gas　sensor,　prepared　by　radio　frequency　reactive　magnetron　sputtering,　showed　a　high　sensitivity　to　ultra-low　concentrations　of　NO　2　in　the　parts　per　billion　range.　X-ray　diffraction,　X-ray　photoelectron　spectroscopy,　and　scanning　electron　microscopy　(SEM)　characterizations　indicated　that　the　MWCNTs　were　affected　by　the　morphology　of　the　SnO2　thin　film　and　the　particle　size.　The　properties　of　the　MWCNT-doped　SnO2　sensor,　such　as　sensitivity,　selectivity,　and　response-recovery　time,　were　investigated.　Experimental　results　revealed　that　the　MWCNT-doped　SnO2　thin　film　sensor　response　to　NO2　gas　depended　on　the　operating　temperature,　NO2　gas　concentration,　thermal　treatment　conditions,　film　thickness,　and　so　on.　The　mechanism　of　the　gas-sensing　property　of　the　MWCNT-doped　Sn22　thin　film　sensor　was　investigated　and　showed　that　the　improved　gas-sensing　performance　should　be　attributed　to　the　effects　between　MWCNTs　(p-type)　and　SnO2　(n-type)　semiconductors.　©　2010　Chinese　Institute　of　Electronics.