Magnetite Fe3O4 nanoparticles of 30-40 nm with a low coercivity are ideal for the hyperthermia applications. The Fe3O4 particles were prepared by oxidizing Fe2+ iron with a weak oxidant NaNO3 in a N2-deaerated aqueous NaOH solution (pH=12-13) at various temperatures below 30 °C. The nucleation features are explored. When the reaction temperature increases, the particle size decreases and the nucleation time shortens. At the two stages of the synthesis reaction, the coercivity is mainly affected by the saturation magnetic flux density and particle size, respectively.