Experimental determination of the free energy barrier to nucleation of amorphous‐to‐crystalline transformation reveals a selection principle for the average scale of crystalline microstructure of partially or fully crystallized materials prepared by crystallization of the amorphous precursors: it is determined by the free energy barrier to nucleation, and minimization of the free energy barrier leads to the formation of the finest crystalline microstructure. It is found that the nucleation at the temperature at which an amorphous alloy is crystallized with the finest crystalline microstructure is an isoenergetic process, the free energy barrier to the nucleation of crystallites at that temperature is contributed only by the entropic change. A model is developed to explain these observations that may provide a unified principle for designing materials with desirable scale of microstructure. ©1995 American Institute of Physics.