Magnetite nanoparticles with super-paramagnetic properties have great potential to achieve advances in fields such as hyperthermia, magnetic resonance imaging and magnetic drug targeting. In particular, magnetic particles less than 50 nm are easily incorporated into cells and generate heat under an alternating magnetic field by hysteresis loss. Various methods of preparing magnetic particles have attracted attention, such as spray pyrolysis, microwave irradiation of ferrous hydroxide, microemulsion technique and hydrothermial preparation technique. In this study, magnetite nanoparticles were synthesized with various molar ratio of Fe2+ and Fe3+ by coprecipitation using spray-guns and dropping syringe. Experiments at different molar concentrations of Fe ions were conducted, which shows the ideal molar concentration of Fe2+ to be 0.5 M for pure magnetite. Both in the spray and drop method, pure magnetite nanoparticles could be synthesized when the molar concentration of Fe2+ was 0.5 M. With increasing the molar ratio of Fe2+, the particle size of the magnetite nanoparticles was increased. The smallest size could be reduced to approximately 7 nm by the spray method. The shape of the synthesized nanoparticles was nearly spherical. The calculated highest loss power by hysteresis losses was 597 W/g, generated with a molar concentration ratio of 0.5:1 (Fe2+:Fe3+).