Epitaxial thin films of (0 0 0 1)-oriented M-type barium hexaferrite, BaFe 12O 19 (or BaM), have been grown on sapphire (0 0 0 1) substrates by plasma-assisted molecular beam epitaxy. In situ monitoring of the evolution of the reflection high-energy electron diffraction (RHEED) patterns showed that the growth of BaFe 12O 19 films began with an unintentional Fe 2O 3 (0 0 0 1) layer with an in-plane lattice parameter of 5.135 Å. Fortuitously, this unintentional Fe 2O 3 layer with a thickness of about 13 nm acts as an important “bridge” layer for the subsequent BaFe 12O 19 growth, and assists in accommodating the large lattice mismatch between BaFe 12O 19 and sapphire. Comparison of the RHEED patterns of the unintentional Fe 2O 3 layers and their corresponding intensity profiles to those of the BaFe 12O 19 films indicated rotation of the BaFe 12O 19 lattice by 30° with respect to Fe 2O 3. The epitaxial relationships for BaFe 12O 19 growth are BaFe 12O 19 [1 1 0 0]//Fe 2O 3 [1 1 2 0]//sapphire [1 1 2 0] and BaFe 12O 19 [1 1 2 0]//Fe 2O 3 [1 1 0 0]//sapphire [1 1 0 0]. The lattice mismatch between the 30°-rotated BaFe 12O 19 films and the Fe 2O 3 bridge layers is less than 1%. The in-plane lattice parameter of BaFe 12O 19 determined from RHEED patterns is about 5.88 Å at the growth temperature of 750 °C.