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 $Fe_3O_4$ interfacial crystallites (50-${AA}$-thick), well aligned $[0.03^circ$full-width at half-maximum (FWHM)] to the sapphire [001] direction. The amorphous precursor was crystallized to epitaxial Ba-ferrite and alpha-Fe_2O_3$grains in two steps; i) the nucleation of crystalline alpha-Fe_2O_3$ phase started at $300^circ{C}$ together with the transformation of the $Fe_3O_4$ crystallites to the alpha-Fe_2O_3$ crystallites, ii) the nucleation of Ba-ferrite phase occurred at temperature above $600^circ{C}$. In the crystallized films irrespective of the film thickness, the crystal domain size of the alpha-Fe_2O_3$grains was about 250 ${AA}$ in the film plane, similar to that of the Ba-ferrite grains.