A synthetic method of fabricating highly crystalline and monodisperse nanoparticles of metals, multi-metallic alloys, monometallic oxides and multi-metallic oxides without a size selection process aredisclosed. A typical synthetic method comprises the steps of, synthesis of a metal surfactant complex from the reaction of a metal precursor and a surfactant, high temperature thermal decomposition ofthe metal surfactant complex to produce monodisperse metal nanoparticles, and completing the formation of synthesized metal, metal alloy or metal oxide nanoparticles by adding a poor solvent followed by centrifuging. For obtaining highly crystalline monodisperse nanoparticles, additional steps are necessary as described in the invention. The resulting nanoparticles have excellent magnetic propert y for many applications.