Using an atom superposition and electron delocalization molecular orbital (ASED-MO) theory, the micro-mechanism of activation of CO adsorbed on Ni(l00) and of poisoning by submonolayer S atom is studied. CO is found to bond to the top site more strongly than on the fourfold site. When CO is adsorbed on the top site, it will get 0.72 e on its antibonding 2π orbital, and its dissociation energy will reduce from 11.1 eV for free CO molecule to 2.15 eV. If CO is adsorbed on the fourfold site, the electrons on its 2π orbital will be 1.22 e and dissociation energy 1.85 eV. It seems likely that CO adsorbed on fourfold site is more active. Our binding energy calculations have verified that one S atom adsorbed on Ni(l00) will inhibit 4 nearest top sites and 4 nearest fourfold sites from adsorbing CO molecule, but will not give any important affection on farther sites. These results support the idea that the dominant effect of adsorbed S atom is "structural effect". These "blockage" effect by the adsorbed S atom, i.e. the reduction of the active sites, and some other factors, result in the poisoning of Ni sur-faces by submonolayer S atom.