CO molecules adsorbed on the Wn clusters are systematically investigated by using density functional theory at the B3LYP/LANL2DZ level.The result indicates that the ground state structures of WnCO clusters are generated when CO molecules are adsorbed on Wn clusters or anionic cluster. We find that among the molecular adsorption states exists mainly the form of end-on type geometry, and that the bridge site adsorption type geometry plays a supplementary role. On the face, the adsorption is a non-dissociative adsorption. The CO bond length increases 0.120–0.123 nm in WnCO cluster (compared with 0.116 nm in free CO molecule), which demonstrates that the CO molecules are activated. The stability analysis shows that W3CO and W5CO clusters are more stable than other clusters; natural bond orbital (NBO) analysis indicate that the interaction between W atom and CO molecule is primarily contributed by hybridization of molecular orbits within CO and 6s, 5d, 6p and 6d orbits of W atoms.