Using transmission electron microscopy we studied changes in the magnetic domain structure during annealing. At room temperature the domain structure is determined by magnetostatic effects. Cross‐tie walls are observed, and the domains vanished at fields of 25 A/cm. After heating the specimen, a two‐stage crystallization behavior occurs. The first stage (140–250 °C) is identified as primary crystallization of polycrystalline &dgr;‐Fe Ni. Further heating (up to 400 °C) shows the formation and growth of (Fe Ni)3B spherulites. The domain structure is determined by the crystallization process. Large spherulites exhibit stripe domains and inside of nonspherulite areas magnetization ripple structures occur. Both of the crystallization centers act as pinning sites for domain walls and are sources of increased coercivity in Fe40Ni40B20amorphous ribbons.