Metallic iron nanoparticles with a crystalline iron oxide shell were synthesized by the thermal decomposition of iron pentacarbonyl [Fe(CO)5] in octadecene in the presence of oleic acid and oleylamine. The effect of different synthetic parameters was investigated in details including the refluxing time and temperature, the injection temperature of iron precursor and the surfactant concentrations. The particles size can be tuned by controlling the injection temperature of iron precursor. Particle composition was adjusted by controlling the refluxing time. Both XRD diffraction and magnetic measurements indicated that these particles are very stable against oxidation which was further evidenced by microstructure analysis.