Recent progress in controlled shape and sizes of nanoparticles using colloidal synthesis technique together with development of in-situ surface science characterization tools such as ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has resulted in enhanced understanding of surface structure of working catalysts. We report AP-XPS study of Ru nanoparticles to monitor surface oxidation states under oxidizing, reducing and CO oxidation reaction conditions. Ru 2.8 and 6 nm nanoparticles were synthesized in the presence of poly(vinyl pyrrolidone) and deposited onto Si wafers by Langmuir-Blodgett deposition as model catalysts. It was found that Ru nanoparticle exhibited mild oxidative and reductive characters, suggesting presence of an oxide overlayer around metallic core. AP-XPS showed that such oxide formation is dependent on Ru nanoparticle size and we found higher oxide formation for Ru 6 nm nanoparticles at all temperatures. The catalytic activity of Ru nanoparticles increases as increasing nanoparticle size in the range of 2 – 6 nm. We suggest that enhancement in catalytic activity for larger nanoparticles are attributed to a stable surface oxide of core-shell nanoparticles. Such in-situ observations of Ru nanoparticles are useful in identifying active state of the catalysts during use and hence may allow for rational catalysts design for practical application.