The oxidation of CO was performed using physical mixture of Ru metal powder and various metal oxide supports after calcination in air and reduction in H-2. Among various supports (CeO2, ZrO2, MgO, Al2O3, TiO2, SnO2,and SiO2), only Ru + CeO2 showed very high catalytic activity for CO oxidation with the light-off temperature below 100 degrees C, which was comparable to Ru/CeO2 prepared by a conventional impregnation method. The optimum conditions for the preparation were loading amount of 2 wt%, calcination at 700 degrees C in air followed by reduction at 400 degrees C in 5% H-2. It was revealed that the high catalytic activity for Ru + CeO2 is attributed to the high dispersion of Ru on CeO2. The original Ru metal powder having diameter of 36 nm was dispersed during the calcination leading into small Ru particles having diameter around 2 nm. Raman spectra indicated that the formation of Ru-O-Ce bond after the calcination is one of the key for the Ru dispersion. The self-dispersion of Ru was not achieved by the calcination in N-2. It was proposed that the self-dispersion of Ru into nano-particles was caused by oxidation of Ru metal into Ru oxide in air, then dispersion of nano-sized Ru oxide by forming Ru-O-Ce bond, and finally conversion of Ru oxide particles into nano-sized Ru metal particles in a flow of H-2. (C) 2012 Elsevier B. V. All rights reserved.