A facile and green method, in which rusty iron (Fe and Fe2O3) and deionized water were irradiated with a household microwave oven as the microwave source to prepare Fe3O4 nanoparticles, without alkali, acid or high-temperature, was proposed in this paper. The as-prepared Fe3O4 nanoparticles attached to the iron surface after the rusty iron was irradiated with microwaves in deionized water for 30 min, which were separated from the surface by ultrasonic wave and magnet. The results show that this method is feasible, and has several advantages, such as convenience, harmless preparation, non-polluting raw materials, and the ability to reclaim and recycle scrap iron.