An electron cyclotron resonance (ECR) plasma enhanced metalorganic chemical vapor deposition (PEMOCVD) system equipped with reflection high-energy electron diffraction (RHEED) has been developed and utilized for epitaxial growth of GaN and AlN on sapphire substrates by PEMOCVD. Since the multicusp cavity-coupling ECR plasma source was adopted to provide active precursors, the growth temperatures were decreased to 600–700 °C and the working pressures were decreased down to the region ≪1 Pa also, which make RHEED monitoring in situ possible for the growth surface. The nitrogen plasma densities Ne∼1.0–3.0×1010 cm-3 with a uniformity ≪±5% over a 10 cm diameter area, the electron temperatures kTe∼2–3 eV, the ion temperatures kTi≤1 eV, and the plasma potentials Vs≪18 V near the substrate holder for the typical film growth conditions: The pressure Po∼3×10-1–8×10-1 Pa and the microwave power Pw∼400–750 W. The experiment results demonstrated important roles of the plasma for sapphire substrate pretreatment, initial nucleation, and epitaxy growth of a large lattice mismatch heterojunction, GaN/(0001) Al2O3 at low temperature. The chemistry and mechanism of hydrogen (H)-plasma cleaning and nitrogen (N)-plasma nitriding, and the 30° rotation of a (0001) nitride plane produced by the nitriding with respect to the (0001) Al2O3 to reduce the lattice constant mismatch are discussed. The epilayers of GaN and AlN with better quality and relatively smooth surface were obtained. The full width of half maximum (FWHM) of a GaN (0002) diffraction peak of x-ray diffraction from a 0.3 μm thick GaN film was 15 arc min and the FWHM of AlN (0002) diffraction peak from a 0.3 μm thick AlN film was 12 arc min. Film surface morphology was observed by atomic force microscopy. © 2004 American Vacuum Society.