The formation mechanism of CH3O by the adsorption and decomposition of CH3OH on clean and oxygen-precovered Cu2O(1 1 1) surface has been investigated with density functional theory method together with the periodic slab models. Two possible formation pathways of CH3O by CH3OH decomposition on oxygen-precovered (O-pre) Cu2O(1 1 1) surface were proposed and discussed. One is the O-H bond-cleavage of CH3OH with H migration to Opre to form CH3O; the other is the C-O bond-scission of CH3OH with CH3 migration to Opre leading to CH3Opre. The calculated results show that the O-H bond-breaking path has the lowest activation barrier 26.8 kJ mol(-1), the presence of oxygen-precovered on Cu2O(1 1 1) surface exhibits a high surface reactivity toward the formation of CH3O by the O-H bond-cleavage of CH3OH, and reduce the activation barrier of O-H bond-cleavage. The C-O bond-breaking path was inhibited by dynamics, suggesting that the O atom of CH3O is not from the oxygen-precovered, but comes from the O of CH3OH. Meanwhile, the calculated results give a clear illustration about the formation mechanism of CH3O in the presence of oxygen and the role of oxygen at the microscopic level. (C) 2010 Elsevier B.V. All rights reserved.