On the basis of density functional theory calculations, we have investigated the structural and electronic properties of the adsorption of one to four CO molecules on the exterior surface of a BeO nanotube (BeONT). It was found that a CO molecule prefers to be attached to a Be atom of the tube surface from its carbon head, releasing energy of 10.47 kcal/mol. Density of states analysis shows that HOMO/LUMO energy gap of the tube is highly sensitive to CO molecule so that it decreased by about 1.76 eV upon the adsorption of one molecule. The E g of CO/BeONT is decreased by increasing the number of the adsorbed CO molecules, indicating that it is also sensitive to the concentration of CO molecules.