As an undesired detrimental byproduct in selective catalytic reduction of NO with NH3 over MnOx/TiO2 catalysts, N2O has received great concern in recent years. Its formation pathways and probable mechanism, especially the sources of N and O, were explored by the application of contrastive experiments and targeted characterization analysis in the work. Experimental results showed that both NH3 oxidation reactions and non-selective catalytic reduction (NSCR) of NO with NH3 accounted for N2O formation. The source of N in N2O formation was not only from NH3 involved in NH3 oxidation reactions but also from NH3 and NO involved in NSCR reactions of NO with NH3, while the source of O was closely related to O-2 and catalyst oxygen. XPS results confirmed the participation of surface absorbed oxygen in N2O formation during all experiments with different O-2 concentrations, and the participation proportion decreased as introduced O-2 concentrations increased. Only a little lattice oxygen acted as the oxidizer and provided O for N2O formation, and with sufficient O-2 in the reactor, the participation of lattice oxygen seemed negligible. Gaseous O-2 was an important source of O in N2O formation, and its role strengthened with the increase of its concentration.