Magnetic powders composed of Fe73.5Si13.5B9Nb3Cu1 were prepared using a ball milling technique, and selected powders were annealed at 823 K in a vacuum. Scanning electron microscopy observations determined the shapes of the powders to be of a flake type. To test the electromagnetic wave absorption properties, Fe73.5Si13.5B9Nb3Cu1 and epoxy composites were fabricated using an electron beam curing technique with powder/epoxy ratios of 20/80, 30/70, 40/60, and 50/50 by weight. The complex permeability and permittivity of the composites were measured using a network analyzer (0.5-8 GHz), and were used to calculate the refection losses as a function of the composite thickness and frequency. The band width of a reflection loss below -20 dB was 700 MHz from 4.23 GHz to 4.93 GHz at 3.8 mm thickness, attributed to the composite containing 50 wt% of powder. For the composite containing 50 wt% of annealed powder, theminimum reflection loss was observed near 8 GHz at 2.8 mm thickness.