Au/MOx(M=metal) were used as effective catalysts for preferential oxidation (PROX) of carbon monoxide in the presence of excess hydrogen. In this study, as the support of Au catalyst, CeO2 which had small crystallite size showed high activity for PROX. XPS studies showed that metallic gold was formed when catalysts were reduced by hydrogen and it was active species. Furthermore, Au particle size should be decreased in order to give high activity being confirmed through XRD and TEM studies. TPR data also showed that the highly active catalyst having well-dispersion had higher reducibility than the others.