Monodisperse cobalt nanoparticles are produced by high-temperature thermal decomposition method. The influence of the size of nanoparticles, evaporation temperature, organic solvent, and concentration on the formation of the cobalt nanoparticles array is studied by transmission electron microscopy and ultraviolet absorption. Preliminary results of the cobalt nonoparticles magnetic properties are presented. These results lay a good foundation for the further study of the physical properties and the application in nanoscale devices of magnetic nanoparticles.