The crystallization of amorphous Ba-ferrite/sapphire(001) thin films was studied in real-time synchrotron x-ray scattering experiments. In the sputter-grown amorphous films, we found the existence of epitaxial interfacial crystallites (50-Å-thick), well aligned [0.03° full-width at half-maximum (FWHM)] to the sapphire [001] direction. The amorphous precursor was crystallized to epitaxial Ba-ferrite and α-Fe₂O₃ grains in two steps; i) the nucleation of crystalline α-Fe₂O₃ phase started at 300℃ together with the transformation of the Fe₃O₄ crystallites to the α-Fe₂O₃ crystallites, ii) the nucleation of Ba-ferrite phase occurred at temperature above 600℃. In the crystallized films irrespective of the film thickness, the crystal domain size of the α-Fe₂O₃ grains was about 250 Å in the film plane, similar to that of the Ba-ferrite grains.