The microstructure of Fe73.5Si13.5B9Nb3Cu1soft magnetic material with a nanocrystalline structure has been investigated by atom probe field ion microscopy. In the as‐quenched amorphous sample, all the alloying elements were found to distribute homogeneously as an amorphous solid solution. In the annealed sample with the optimum soft magnetic properties, we found that two phases were present. One was Fe‐Si solid solution and the other was the B‐ and Nb‐enriched region with less Si content. Although some uncertainty remains regarding Cu, the present result is consistent with an early speculation based on the lattice parameter measurement.