The reaction mechanism in a NOx-trap type catalyst for a partial lean burn engine is discussed using a thermodynamic calculation approach. The thermodynamic calculation and catalyst characterization suggest the following reaction mechanism; During lean operation, NO2, which was formed from NO oxidation reaction by precious metals, reacted with M-Carbonate(M: NOx Trap Material such as alkali earth elements) to form the corresponding M-Nitrate on the catalyst. When the A/F switches to rich, M-Nitrate decomposed to M-oxide and NO2. Released NO2 was purified to N2. M-oxide reacted with CO2 to form M-Carbonate. Thermodynamic calculation further suggested that NOx trap performance depended on the basicity of added NOx trap material, and evaluation results of the performance in Pt/Rh type catalyst supported this tendency.Furthermore, impacts of catalyst formulations and reaction parameters on NOx trap performance were investigated for identification of the NOx trap reaction mechanism. Investigations assumed that NOx trap performance of the catalyst was dominated by the reaction equilibrium in NOx trap process.As a result, the reaction mechanism required the catalyst to have: 1) effective NO2 formation, 2) strong basicity in NOx trap material, and 3) efficient use of trap material. On the basis of these findings, an advanced NOx trap catalyst has been developed.