Synthesis of magnetite Fe3O4 nanoparticles functionalized with polyethylene glycol was carried out by co-precipitation method using a very small amount of organic substances to obtain a highly monodisperse form of nanoparticles, and the structure of obtained Fe3O4 nanoparticles was studied by physical methods. During the synthesis of magnetic Fe3O4 nanoparticles surrounded by a polymer, it has been determined that by adjusting the amount of used polymer, the nature of the precipitating agent, the concentration of the initial precursors and the temperature it was possible to synthesize and stabilize monodisperse Fe3O4 nanoparticles with a high degree of purity, uniform in size