Combining experiments and first-principle calculations, mechanism of ozone detection on Cu2O(111) surface is proposed. Results show that O3 is adsorbed without dissociation with Eads=−2.89eV and is completely dissociated in O2 plus oxygen adsorbed with total enthalpy ΔH=−3.01eV. These two reactions leads to a shift in the level Fermi of about 0.45eV, which induces a p-doping. Thus the reaction of dissociation of O3 on Cu2O(111) surface leads to a decrease in electrical resistance, as experimentally observed.