The interaction of CO and CO2 with nickel has been studied comparatively using the ab initio method. The results show that the carbon-oxygen bond is weakened when CO interacts with nickel, and the weakening is more obvious in the bridged adsorbed model than that in the linear one. The calculated vibrational frequencies are close to those found experimentally. Three interaction modes of CO2 with a Ni2 small cluster are found to be possible in which the most stable one has a bidentate structure, i.e. two oxygen atoms bonded to two adjacent nickel atoms, respectively. The weakening of the carbon-oxygen bond is more remarkable in the CO2 adsorbed model than in the CO adsorbed one. The Ni2 cluster structure suitable for CO2 is different from that for CO except that the Ni-Ni interatom distance in the CO bridged adsorbed species is significantly close to that in CO2 bidentate species.