This article aims to synthesize and character cobalt nano-particles through microwave polyol process. At first, cobalt acetate is used as precursor to synthesize cobalt nano-particles by microwave assisted ethlylene glycol in the presence of polyvinyl-pyrrolidone (PVP) as capping agent. As measured from ion-coupled plasma (ICP) and thermo-gravimetric analysis (TGA), the yields of as-synthesized cobalt particles in average diameters of 40 and 81 nm verified by transmission electronic microscopy (TEM), are 71 and 85%, respectively; and the structure of as-synthesized cobalt nanoparticle is a face-center cubic structure analyzed by X-ray diffraction (XRD). Finally, the as-synthesized cobalt nano-particle modified by oleic acid is dispersed in kerosene to character magnetic property. As measured by vibration sample magnetometer (VSM), the saturation magnetisms of cobalt nano-particles with average diameters of 40 and 81 nm are 39.1 and 29.5 emu/g, respectively, in solution. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 1383-1389, 2009