Transition metal oxides are desirable alternatives to precious metals based catalysts for CO oxidation since they are less costly. Several nanosize iron oxides were prepared by using Headwaters Technology Innovation (HTI) NxCat® colloidal chemistry nanotechnology. These catalysts were applied on CO oxidation and showed better performance than bulk iron oxide, and comparable performance to catalyst with 100 ppm Pd loading. The particle size of iron oxide veries with calcination temperature. When anneal temperature is below 300oC, the particle size of iron oxide is less than 10 nm based on TEM and XRD characterizations and is mainly existed as Fe3O4. The size grows up to over 100 nm and the main phase is Fe2O3 while anneal temperature reaching 500oC. At SV=600,000 h-1, the advantage of nanosize iron oxide (3-5 nm) in onset activity is over 100oC compared to bulk iron oxide.