Abstract: Monodisperse iron nanoparticles were prepared by thermal decomposition of iron carbonyl at low temperatures of 160-180℃ in kerosene.The synthesized iron nanoparticles were spherical and their average size decreased from 11.2 to 8.6 nm as increasing the initial concentration of iron carbonyl from 0.296 to 0.444 mol/L.The synthesized iron nanoparticles are amorphous,but the surface of the particles is easily oxidized and is spinel structure iron oxide by HRTEM.