To derive an effective technique for reducing a greenhouse gas nitrous oxide (N2O) emitted from automobiles, we prepared experimental three-way catalysts carrying various types and quantities of precious metals, and investigated their N2O generation and conversion characteristics. In view of previous reports on increased N2O emissions from in-use automobiles, we deactivated the catalysts in a rapid aging test, and observed the effect of catalyst deactivation on N2O generation and conversion by the catalysts.We found that the concentrations of generated N2O decreased as the quantities of precious metals carried by catalysts were decreased and that, accordingly, these low-carrying catalysts are more advantageous from the standpoint of reducing N2O generation. However, the concentrations of generated N2O increased as the catalysts were deactivated. In palladium three-way catalysts, the N2O generation peak temperature shifted higher due to deactivation, and N2O generation in the high temperature range also increased.Regarding the decomposition of N2O by three-way catalysts, high precious metal carrying catalysts indicated higher N2O conversion rates. However, their N2O conversion rates were reduced by catalyst deactivation so that the N2O which remained unconverted increased in the temperature range of the most frequent catalyst operation. Thus, catalyst deactivation resulted in an increase in N2O emissions from automobiles.