This paper presents the research results on the synthesis of Fe3O4 magnetite nanoparticle aqueous suspension by coprecipitation from solution containing both ferric and ferrous iron ions. The process parameters such as the Fe2+/Fe3+ ratio, synthesis temperature and the effects of surfactants were investigated. The properties of obtained products were characterized by X-ray diffraction, magnetic hysteresis loops, transmission electron microscopy, etc. The results show that synthesized suspension consists of Fe3O4 nanoparticles of 3-5 nm, dispersion stable and have good magnetic property (the saturation magnetization ~ 80 emu/g).