Abstract The crystallization behavior of the amorphous Fe 91− x W x B 9 (0< x<14), the microstructure and the soft magnetic properties of the nanocrystalline Fe–W–B alloys have been investigated. It was found that the primary crystallization precipitates consist of mixture of the bcc-Fe and bct-Fe 3B phases. The tungsten addition makes grain size decrease. The grain size of the primary Fe 91− x W x B 9 (6⩽ x⩽12) alloys was evaluated to be 18–9 nm. The small grain size seems to reflect the small growth rate of the primary precipitates resulting from re-distribution of the W element between the primary bcc-Fe, bct-Fe 3B and residual amorphous phases, which is due to their small solubility, large cohesive energy and small diffusivity to α-Fe. The best soft magnetic properties have been obtained for the Fe 91− x W x B 9 (6⩽ x⩽12) optimally annealed (900< T a<936 K) alloys. These nanostructured alloys show high magnetic flux density of 1.58–1.76 T, high permeability of 21.000–33.000 at 1 kHz and zero magnetostriction, simultaneously. This behavior is thought to result from the exchange magnetic coupling mechanism among the different ferromagnetic phases containing in these optimally annealed alloys. It is concluded that the nanocrystalline Fe–W–B optimally annealed alloys (6–12 at.% W) are suitable materials for core pole transformers.