Fe55Ni45 fiber has been fabricated by magnetic-field-induced thermal decomposition of iron pentacarbonyl Fe(CO)(5) and nickel tetracarbonyl Ni(CO)(4). Fe55Ni45 fiber presents the face-centered-cubic (FCC) structure (gamma-phase) characterized by X-ray diffraction (XRD) and fibrous morphology observed by scanning electron microscope (SEM). It is shown that with the increase of induced magnetic field, the diameter of the fiber can be increased from 5 to 8 mu m and its corresponding aspect ratio is decreased from 60-80 to 20-40. The complex permittivity and permeability of Fe55Ni45 fiber-filled composites are measured in x-band (8-12 GHz) with the transmission and reflection waveguide method. The results show that Fe55Ni45 fiber of smaller diameter (5 mu m) and higher aspect ratio (60-80) exhibits higher complex permittivity and permeability. The reflection loss (RL) measurement reveals the thin composites filled with Fe55Ni45 fibers possess low RL value. (c) 2005 Elsevier B.V. All rights reserved.