Background： Chemical sensors that work as electronic noses have attracted extensive attention, yet research about sensor application of C-doped boron nitride nanotubes is still rare. Purpose： In order to realize the gas sensing properties of boron nitride nanotubes （BNNTs） and C-doped BNNTs for some small gas molecules, such as NO2, O2 and F2, we calculated the adsorption property of those gas molecules. Methods： The gas sensing properties of C-doped BNNTs for some small gas molecules have been investigated by using the density functional theory. Results： The interaction distance between gas molecule and BNNT and adsorption energy of gas molecule on the BNNTs and the C-doped BNNTs are obtained by density functional theory （DFT） calculation. We also calculated the electron densities of states for an 02 molecule or a NO2 molecule or a F2 molecule to adsorb on the different positions of C-doped BNNTs. Conclusion： The calculated results show that BNNTs present high sensitivity to the gaseous NO2, O2 and F2 molecules. The chemical reactivity of BNNTs has been changed by carbon molecules, and C-doped BNNTs can improve the interaction between the gas molecules and the BNNTs.