Abstract: CO adsorption, CO dissociation and C－H and C－C bond-making reactions on Cu-covered Fe(100) have been computed within the framework of periodic density functional theory. Compared to the clean Fe(100) surface with strong CO activation, Cu monolayer-covered Fe(100) surface has weaker CO adsorption energy, lower CO activation degree; and ultra higher CO dissociation barrier (2.4eV). Under H co-adsorption, CO dissociation is favored kinetically on Fe(100). On the Cu-monolayer covered Fe(100), co-adsorbed H largely facilitates CO dissociation via the formation of CHO. The energy barrier decreases to as low as 0.92eV. It is also found that C－H and C－C bond-making reactions of Cu monolayer-covered Fe(100) surface have lower activation barriers and are more exothermic than on the clean Fe(100) surface.