Nanostructured Fe40Ni38Mo4B18 specimens with a wide range of different nanostructures can be produced by annealing the amorphous alloy in the temperature range from 700 K to 1060 K. The nanostructured Fe40Ni38Mo4B18 specimens prepared between 700 K and 760 K have a microstructure of the 4-9 nm fee (Fe, Ni) solid solution crystallites embedded in the amorphous matrix. The nanostructured Fe40Ni38Mo4B18 specimens obtained between 780 K and 910 K show a polycrystalline microstructure of the major cubic (Fe, Ni, Mo)(23)B-6 phase with grain sizes from 20 nm to 40 nm and the minor fee (Fe, Ni) solid solution crystallites of about 10 nm. The nanostructured Fe40Ni38MO4B18 specimens produced between 930 K and 1060 K represent a polycrystalline microstructure of the 12-25 nm fee (Fe, Ni) solid solution crystallites and the 45-240 nm cubic (Fe, Ni, Mo)(23)B-6 crystallites.