Magnesium oxide was grown by gas source molecular beam epitaxy on (0001) oriented metalorganic chemical vapor deposition n-GaN using elemental Mg and atomic oxygen supplied from an electron cyclotron resonance plasma source. X-ray diffraction (XRD) indicated that the oxide was single crystal for TSUB=350 °C and mostly polycrystalline for TSUB=100 °C. Reflection high energy electron diffraction suggests that the films deposited at the lower temperature begin with a single crystal nucleation layer then quickly become polycrystalline. For both growth temperatures, the magnesium oxide was highly textured toward the (111) direction, with the polycrystalline samples showing a broader XRD peak but smoother surfaces. Single crystal MgO grown at 350 °C had high current leakage, prohibiting electrical measurements. A breakdown field of 2.3 MV/cm and an interface state density of 4×1011 cm-2 eV-1 were measured for the polycrystalline (TSUB=100 °C) magnesium oxide/GaN heterostructure. © 2003 American Vacuum Society.