The geometry, electronic structure, magnetism, and adsorption properties of one CO molecule on the Mn(N) (N = 2-8) clusters have been investigated based on the density functional theory (DFT) with the spin polarized generalized gradient approximation. It is found that the CO molecule adsorbs on the atop site for N = 2, 4, 7, 8 and on the bridge site for N = 3, 5, 6. The results of the calculated second-order energy differences of bare Mn(N) cluster indicate that the Mn(3), Mn(6), and Mn(8) clusters have relatively low stability. However, their corresponding CO complexes possess high adsorption ability implied by the C-O bond length, vibrational frequency, adsorption energy, and the charge transfer between the CO molecule and the clusters. Compared with bare Mn clusters, the adsorbing of a CO molecule enhances the magnetic moments of the Mn(N) clusters for N = 4, 6-8.