Iron oxide (Fe3O4) nanoparticles were synthesized by a facile microwave combustion method. X-ray diffraction and energy dispersive X-ray spectroscopy results showed that the as-prepared product was pure Fe3O4 without any impurity. The mechanism for the formation of Fe3O4 by microwave combustion method is also discussed. Microwave combustion produced sufficient energy for the formation of Fe3O4, because of its homogeneous distribution within the raw materials. This results in the formation of nanoparticles, early phase formation and different morphologies within few minutes. Magnetic analysis revealed that the Fe3O4 nanoparticles had ferromagnetic behavior at room temperature with saturation magnetization of 66.12 emu/g. © 2013 The Korean Society of Industrial and Engineering Chemistry.