The present work has been undertaken to improve the thermal stability of an RhPt-NaTi/Al2O3 catalyst, which is one of promising lean NOx trap catalysts for practical NOx conversion. For this purpose, we mainly directed our attention to the component of Na that acts as NOx trapping sites and intended to improve the thermal stability of this Na species by using various additives. After screening of various RhPt-Na alpha/Al2O3 (alpha = Ti, Si, Mg, Ca, Mn, Co, Ni, La, and Nd) catalysts, Mn addition was found to be effective for improving thermal stability of the catalyst. Moreover, the modification of Al2O3 support with Ce additive was indicated to be effective, and the most active and thermally stable catalyst was RhPt-NaMn-Ce/Al2O3, which was able to function actively even after a severe heat treatment at 800 degrees C. The reasons for the high thermal stability of this catalyst were examined by evaluating NO oxidation activity of a model catalyst including Mn and characterizing features of the RhPt-NaMn-Ce/Al2O3 catalyst with FTIR of adsorbed NO and chemisorption of CO and CO2. An important factor is that the addition of Mn prevents the Na species from undesired sintering during heat treatments, resulting in a larger surface area of exposed Na species, namely NOx trapping sites. This catalyst can also function in a commercial vehicle engine under lean-burn conditions and should be a promising catalyst for practical use. (C) 2010 Elsevier B.V. All rights reserved.