In our test, the CO/NOx ratio obtained in Euro IV diesel engine exhaust gas during the ESC/ELR+ETC mode test was no less than 16 but, UHC/NOx ratio was as low as 5.5. From this, CO would be preferred as a SCR reductant than UHCs for passive-DeNOx system which was operated without equipment providing additional reductant to the system. Two kinds of combined catalyst system were designed. The one is the system using double layer of separated catalyst (WGSR → DeNOx) and the other is single layer of mixed catalyst (WGSR + DeNOx). In the system using Pd/TiO2/Al2O3 catalyst alone, conversion of NOx into N2 did not occur. This result was expected that NOx adsorbed on the catalyst instead of conversion to N2. As the concentration of hydrogen increased, NOx→N2 conversion and total NOx conversion were increased. This implied that H2 plays an important role in reducing NOx with CO. In the DeNOx and WGSR mixed system at 200°C, homogeneously mixed system showed higher DeNOx activity (37%) and NOx into N2 conversion (25%) than serially packed system. In serially packed catalyst system, H2 generated by WGSR catalyst reacted with O2 before acting as reductant for DeNOx system. On the other hand, H2 acted as a reductant more effectively in homogeneously mixed catalyst system.