The adsorption of CO on the Nin cluster surfaces has been systematically investigated by density functional theory. The result indicates that the lowest energy structures of NinCO are generated with CO being adsorbed on Nin clusters, and the lowest energy structures of Nin clusters are not changed by adsorbing CO molecule， The chemisorption of CO on Nin cluster surfaces belongs to non-dissociative adsorption. The increased theoretical CO bond length of 0.1180-0.1214?nm （compared with 0.1138?nm） demonstrates the activation of the CO bond. Natural bond orbital analysis shows that the interaction between Ni atoms and CO molecule is primarily contributed by the hybridized molecular orbital within CO and 3d, 4s, 4p orbital of Ni atoms.