Water adsorption and decomposition on the Co(0001) surface has been systematically studied by spin-polarized density functional theory calculations and atomic thermodynamics. H2O adsorption mechanism has been analyzed by partial density of states. The possible structure of adsorbed H2O molecules comprised of monomer-hexamer have been investigated and the phase diagram shows that only two configurations are stable thermodynamically: clean Co(0001) surface and H2O hexamer adsorption. The competition between the ability of a H2O molecule to bond with the substrate and its ability to act as a H-bond acceptor leads to the symmetry-breaking bond alteration in the hexamer structure. In addition, the interaction among adsorbed H2O molecules can help stabilize adsorption configurations by forming H-bonds. Presence of O species has a great influence on the decomposition of water and can significantly lower the activation barrier of H-OH bond cleavage. [GRAPHICS] .