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An Fe-based nanocrystalline alloy powder is important for application in microwave absorption, and the particle size has a critical impact on the electromagnetic microwave parameters. Therefore, it is necessary to study further the effects of the particle size on such parameters and improve the microwave absorption performance of Fe-based nanocrystalline powers. In this study, Fe-B-P par-ticles were prepared through a synthetic approach con-sisting of an aqueous chemical reduction and a ball milling treatment. We investigated the effects of ball milling on the microstructure and electromagnetic properties of Fe-B-P particles. The experimental results indicate that the Fe-B-P particles synthesized through an aqueous chemical reduction are amorphous spheres. Fe-B-P particles with an original particle size of 200-1200 nm can be milled into an irregular shape with the size reduced to﹤500 nm after 0.5 h of ball milling, and subsequently, the particles become smaller with increases in the milling time, with traces of Fe2O3 generated on the particle surface. The results of the M(o)ssbauer spectra show that a portion of the small particles demonstrate a superparamagnetic property. The volume proportions of the superparamagnetic com-ponent increase from 13.1 to 15.8％as the treatment time increases. We measured the permittivity and permeability spectra of Fe-B-P particles within the frequency range of 2-18 GHz. The reflection loss (RL) is -10 dB for an absorber thickness of 1.7-5.0 mm. The RL is-20 dB for an absorber thickness of 1.9-2.7 mm. The microwave absorption properties of samples with the same thickness are improved with an increase in the treatment time and are shifted to a higher frequency, which will broaden the bandwidth of the absorption as well.