N2O formation over Cu-SSZ-13 during selective catalytic reduction (SCR) of NOx with NH3 under different conditions was experimentally investigated based on a flow reactor and in situ diffused reflectance infrared Fourier transform spectroscopy experiments. The results show that the NSCR and the nonselective NH3 oxidation are the major pathways for N2O formation over Cu-SSZ-13. N2O formation at low and high temperatures usually occurs via different mechanisms. The N2O formation at low temperatures is mainly ascribed to the decomposition of NH4NO3, which mainly forms on Cu(OH)(+), while at high temperatures, N2O is formed because of the NH3 oxidation by O-2 and NOx. Besides, the effect of ANR, NO2/NOx ratio, and O-2 and H2O concentrations on N2O formation were also studied. They all affect N2O formation, and the effects on the two mechanisms are totally different at low and high temperatures.