Abstract: Co3O4 nanoparticles of controllable size were synthesized via a variant microemulsion method combined with hydrothermal treatment. With this improved method, Co3O4 nanoparticles with uniform shape and narrow size distribution were controllably synthesized. By varying the surfactant and water ratio, particles of around 27.97 nm, 42.88 nm and 48.99 nm were synthesized. These nanoparticle catalysts behaved differently at low and high temperature. These samples showed high activity for CO oxidation and 100% CO conversion was achieved at 30 degrees C. However, the catalysts gradually lost their activity after about 100 min. Our research found that CO2 had a huge influence on the stability of catalysts at low temperature. The relationship between stability at 30 degrees C and ratio of Co3+/Co2+, as well as particle size and desorption rate of CO2, was also thoroughly investigated in this work.