Geometric structures of CoBen(n=1—12) clusters are optimized using the generalized gradient approximation (GGA) density functional theory. The energy, vibrational frequency and magnetism are calculated. The results indicate that the spin multiplicities of the ground-state clusters are 2 and 4. Furthermore, the investigated magnetic moments confirm that the Co atomic magnetic moments of CoBen(n=1—12) clusters display an odd-even oscillation feature. In addition, the Co atomic magnetic moments of CoBe6 is the smallest of all clusters due to the strong hybridization between the 4s, 3d state of Co and 2s, 2p state of Be and short Co-Be average bond distance and low symmetry. By analyzing the properties of electrons, it is concluded that doping of impurity increases the stability and the chemical activity of Be cluster. It is found that CoBe5 and CoBe10 clusters are more stable than the neighboring ones.