Synthesis of monodisperse Cobalt nanocrystals by high temperature diol reduction methods in the presence oleic acid and triocethylphosphine stabilizers were reported.The mean size of the Co nanocrystals is about 14±19nm and a standard deviation of the diameter is less than 2%.The structure and the surface valency state of the Co nanocrystals were measured by XRD,Uv-vis,XPS and TEM.The monodisperse Cobalt nanocrystals can be assembled to form a two-dimensional hexagonal close-packed structure.