Single walled carbon nanotubes (SWNTs) have been shown to be highly sensitive gas sensors. However, attaching functional groups with selective sensing functions on nanotubes without destroying the intrinsic electronic property of the nanotubes is still challenging. Here, we report a new method of coating SWNTs with a thin layer of SiO2 using 3-aminopropyltriethoxyysilane as coupling layers. The thickness of the SiO2 could be controlled at about 1 nm. The coating of SiO2 on SWNTs was confirmed by burning the SWNTs in air. The effect of 3-aminopropyltriethoxyysilane was also discussed.