We have reported a LaMnO3-based, Pt free lean NOx trap (LNT) catalyst shows promising NOx reduction activities, but the mechanism of NOx reduction or the role of the perovskite over this catalyst has not been addressed. This paper presented our investigation on the mechanistic aspects of NOx adsorption and reduction over several LaMnO3-based LNT catalysts. NO can be effectively oxidized to NO2 on LaMnO3 at 350 degrees C, while little NOx (NO + NO2) can be stored verified by the following temperature programmed desorption experiment; under the same conditions, a significant amount of NOx (41 mu mol NO/g washcoat) were stored over BaO/Al2O3/LaMnO3 indicating NOx are adsorbed mainly on the BaO/Al2O3. The temperature programmed surface reaction with CO demonstrated that although it can store NOx during the lean period, failed to reduce NOx by CO over the BaO/Al2O3/ LaMnO3 catalyst. After adding Pd into BaO/Al2O3/LaMnO3, NOx can not only be stored during the lean conditions but also reduced. It could be concluded that the Pd catalyst, not perovskite, is responsible for NOx reduction during the rich conditions.