It is shown that the full isothermal crystallization analysis of Fe80B20amorphous alloy must include not only a crystal nucleation‐and‐growth process but also a grain‐growth process and that these two processes are separated in time during isothermal annealing. The parameters characterizing the grain‐growth process have been found by the fitting calorimetrical signal with the one expected from the normal grain‐growth theory. Transmission electron microscopy and selected area electron diffraction seem to confirm our results since they show that there is a good agreement between the average grain radius of crystallization products seen in the micrograph and the one calculated from the obtained parameters.