By using density functional theory, the electronic response of a BC3 nanotube (BC3NT) to CS2 molecules was investigated. It was found that: (1) the CS2 adsorption on the tube is site-selective and it prefers to be adsorbed on a hexagonal ring of the tube which has six carbon atoms, liberating energy of about 34.36kcal/mol; (2) the HOMO–LUMO energy gap (Eg) of the tube is significantly decreased from 2.37 to 1.70eV upon the CS2 adsorption; (3) the Eg of the tube is more decreased by increasing the number of the adsorbed CS2 molecules; (4) the BC3NT may transform the presence of CS2 molecule into an electrical signal, and therefore it may be potentially used in CS2 sensors. Also, the response of the tube may be dependent on the concentration of the CS2 molecules.