An on-line, continuous IR-based N2O measurement system has been developed by combining with a variable multipath “White cell” to avoid a huge amount of the well-known artifact errors in actual N2O concentrations determination when analyzing grab samples taken from stationary sources, such as fossil fuel-fired power plants. For solving the problems confronted in earlier stages of this study, the gas cell had to have modifications of the feed through of gas sample flows, the multilayer coatings of stainless steel mirrors, and the thermal efficiency to provide high cell inner temperatures in flowing gas samples. These modifications allow good tolerance of the gas cell to gases and chemicals, such as NOx and NH3, and NH4NO3 driven from them, and its usage for a long lifetime even under harsh conditions. They also offer excellent performance not only in directly determining the extent of N2O formation during the course of NH3-SCR reaction over a sample of a commercial V2O5-WO3/TiO2 catalyst, but also in simultaneously monitoring changes in concentrations of NO, NO2 and NH3 during the reaction. Each reference peak was chosen in gasphase spectra for N2O, NOx, NH3 and H2O, and CO2 as a possible interference, and the modified gas cell was finely tuned to obtain their spectra with a high resolution under optimal operating conditions. The catalyst gave significant amounts of N2O formation at reaction temperatures greater than 350 ℃, and attention should be paid to the possibility of N2O production from commercial NH3-SCR deNOx processes with V2O5/TiO2-based catalysts.