Recent progress in colloidal synthesis of nanoparticles with well-controlled size, shape, and composition, together with development of in situ surface science characterization tools, such as ambient pressure X-ray photoelectron spectroscopy (APXPS), has generated new opportunities to unravel surface structure of working catalysts. We show an APXPS study to investigate catalytically active species on Ru nanoparticles under oxidizing, reducing, and CO oxidation reaction conditions. Ru nanoparticles 2.8 and 6nm in size were synthesized in the presence of a poly(vinyl pyrrolidone) polymer capping agent and deposited onto a flat Si support as two-dimensional arrays, using the Langmuir-Blodgett deposition technique, as model catalysts. It was found that both sizes of the Ru nanoparticles are covered with a surface oxide layer. Mild oxidative and reductive characteristics suggest an ultra-thin shell of surface oxide around the Ru metal core, the thickness of which is found to be dependent on the nanoparticle size. The smaller 2.8nm Ru nanoparticles were oxidized to a larger extent than the larger 6nm nanoparticles, within the temperature range of 50~200oC. The stability of these active core-shell type Ru nanoparticles is found to increase with nanoparticle size. The catalytic activity of Ru nanoparticles increases with increasing nanoparticle size, within the 2.1 to 6nm range. Thus, particle size dependence of Ru nanoparticles for catalytic CO oxidation can be correlated with enhanced stability of the core-shell type Ru oxide as particle size increases.