By means of density functional theory, the ammonia decomposition reactions catalyzed by Ni-13, Cu-13, and Ni12Cu clusters have been studied and compared. We firstly investigated the structural stability of these clusters, and then systematically investigated their ammonia decomposition activity by analyzing the adsorption property of reaction intermediates and the relative energy diagram. The results show that the adsorption energy of reaction intermediate N on Ni12Cu cluster is -5.93 eV, which is very close to the optimal value (-5.81 eV) of ammonia decomposition volcano curve. The reaction energy diagram shows that the dehydrogenation of NH intermediate is the rate-determining step for these clusters due to the positive reaction heat. Furthermore, the catalytic property of Ni12Cu cluster is interpreted by density of states. It indicates that the adsorption energies of reaction intermediates mainly depend on the d-band center of the clusters.