The evolution of the microstructure during the crystallization of amorphous Fe 80 B 20 alloys has been investigated by small‐angle neutron scattering (SANS) and transmission electron microscopy (TEM). Samples with an increasing degree of crystallization were obtained by performing different isothermal heat treatments on melt spun amorphous ribbons. The TEM measurements indicate that the crystallization occurs by nucleation and growth of Fe 3 B spherulites which include small finely divided α‐Fe acicular crystallites. On the basis of the TEM results a model for the quantitative analysis of the SANS data has been formulated. The results show that, due to the high growth velocity of the nucleated particles, only their number increases with increasing annealing time, while their size and internal structure is not dependent on the degree of crystallization. © 1996 American Institute of Physics.