Multi-walled carbon nanotubes (MWNTs) confined Ru catalysts were prepared by a modified procedure using ultrasonication-aided capillarity action to deposit Ru nano-particles onto MWNTs inner surface. The structure properties of MWNTs supports and Ru catalysts were extensively characterized by XRD, TGA, H-2-TPR, XPS, TEM, FTIR and Raman spectra. The catalytic performance in the preferential oxidation of CO in a H-2-rich stream was examined in detail with respect to the influences of Ru loading, MWNTs diameter, various pretreatment conditions, and the presence of CO2 and H2O in the feed stream. In contrast with Ru catalysts supported on MWNTs external surface and other carbon materials, the superior activity was observed for the MWNTs-confined Ru catalyst, which was discussed intensively in terms of the confinement effect of carbon nanotubes. The optimized catalyst of 5 wt.% Ru confined in MWNTs with diameter of 8-15 nm can achieve the complete CO conversion in the wider temperature range and the favorable stability at 80 degrees C under the simulated reformatted gas mixture, which proves a promising catalyst for preferential CO oxidation in H-2-rich stream. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.