To find a clue to reduction techniques for Nitrous Oxide (N2O) emission from three-way catalyst equipped vehicles, four test samples of three-way catalysts with typical noble metal compositions were fabricated by way of experiment and their N2O formation characteristics have been experimentally studied. Then, these catalyst samples were conditioned artificially by aging with real automotive exhaust gas and the N2O formation characteristics after aging has been also observed.As results, catalyst temperature zones and concentration levels of N2O formation varies greatly by the catalyst composition. In general, a catalyst with lower metal content showed lower N2O mass emission at both fresh and after aging conditions. The tendency of the increase in N2O mass emission due to the deterioration is also different among the tested catalyst samples.It was also clarified that the main cause of N2O increase with aged catalyst was the shift of maximum N2O formation temperature zone to the higher regions. Catalyst deterioration effects on N2O emission of a vehicle were predicted using model gas experiment and catalyst temperature distribution of a passenger vehicle driven on a chassis dynamometer with various test procedures.