The fundamental characteristics of NO reduction and the formation of nitrogen compounds such as N2O, NH3 and HCN were investigated on a metal-honeycomb-supported three way catalyst. Experiments were carried out in a quartz glass tube reactor by flowing simulated gas mixtures in the NO/H-2/Ar, NO/CH4/Ar and NO/CO/Ar systems. In the NO/H2/Ar system, NO reduction began at the lowest temperature of all the systems examined. In any system, the conversion ratio of NO reduction was over 90% at gas temperature higher than 900 K. However, depending on temperature, NO reduction resulted in nitrogen-compound formation as NH3 in the NO/H-2/Ar system, HGN in the NO/CH4/Ar system, and N2O in all systems. In particular, N2O formation was dominant at relatively low temperature. The effect of the coexistence of the oxygen on NO reduction was also examined and a sufficient conversion was observed if the O-2 concentration was less than 1 vol%.