Two different approaches to obtain monodisperse Fe nanoparticles (NPs) in a wide range of average diameters based on thermal decomposition of Fe(CO)₅ were developed; the first is thermal decomposition of a Fe(CO)₅-oleylamine (OAm) reacted precursor, and the second is a method of Fe(CO)₅ slow injection. OAm as a surfactant strongly adsorbed on an Fe atom in the thermal decomposition of a Fe(CO)₅-OAm reacted precursor and restricted the particle growth process. As a result, small NPs 2.3 nm in diameter were obtained. Moreover, it was possible to control the NP diameter in the range from 2.3 to 10 nm by changing the reaction temperature and the time for precursor preparation. However, large NPs over 20 nm in diameter were obtained with the method of slow injection by promoting the process of particle growth. Controls for the density of dissolved Fe atoms and Fe NPs in a reaction solution were required to accelerate particle growth without nucleation. Modifications to the injection rate and/or thermal decomposition temperature of Fe(CO)₅ thus enabled us to control the NP diameter in the range from 12 to 21 nm.