Amorphous FexB100-x (64 <= x <= 91) nanostructures with tunable boron contents and crystallinities were fabricated through a wet chemical reduction method by simply altering the adding rate of iron salt. XRD patterns and Mossbauer spectra show that the amorphous components in the FexB100-x nanostructures increase as the boron content are improved. Magnetic properties reveal that the saturation magnetization and coercivity of the samples strongly depend on the boron contents, and the amorphous one performs lower coercivity and remarkable soft magnetic behavior. The high -frequency electromagnetic characteristics of the representative crystalline and amorphous FexB100-x nanomaterials were investigated. In comparison with the crystallized FexB100-x nanostructures, the amorphous sample displays improved complex permeability and reduced complex permittivity, which is mainly originated from the lower anisotropy field and high resistivity of the amorphous phase. The microwave absorption properties show that the amorphous sample has a maximum reflection loss of -39.4 dB at 4.2 GHz with a matching thickness of only 2.0 mm. Compared with the crystallized one, the amorphous sample shows a dramatic enhancement of microwave absorption properties, which is mainly due to that its relatively high permeability and low permittivity are beneficial for the impedance matching. These results suggest that the amorphous nanostructures are good candidates as high efficient microwave absorbing materials. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.