For future removal of NOx by catalysts, low-temperature NO reduction is desired. Results confirmed that a drastic improvement of catalytic activity by H2O on NO-CO-O-2 reaction over Pd/La0.9Ba0.1AlO3-delta catalyst at the low temperature of 473 K or below. In a humidified condition, NO reaction with CO on Pd/La0.9Ba0.1AlO3-delta proceeded without being affected by competitive adsorption of NO and CO, whereas that on Pd/Al2O3 was inhibited by strong adsorption of CO on a Pd surface. From in-situ DRIFTS measurements, results showed that nitrite species on the support react with CO adsorbed onto Pd at the periphery of Pd particles and that carbonate species accumulated on Pd/La0.9Ba0.1AlO3-delta are removed rapidly in a humidified condition. Although NO reduction proceeds dominantly on the Pd surface in a dry condition, supplied steam promotes desorption of the surface carbonate to advance the reaction of nitrite with CO for de-NOx in a humidified condition. This mechanism occurs specifically on Pd/La0.9Ba0.1AlO3-delta by virtue of the lattice oxygen and oxygen vacancy on La0.9Ba0.1AlO3-delta.