We have investigated what are the key issues for obtaining high-quality GaN films by atmospheric two-flow metalorganic vapor phase epitaxy, in which one main flow carries a reactant gas parallel to the substrate and the other subflow carries an inactive gas nearly perpendicular to the substrate. There are three key issues: (1) The uniformity of temperature on the susceptor is good enough. (2) The Substrate is leveled with the accuracy of 0.01. (3) The subflow tube is adjusted to an optimum tilt angle with the accuracy of 0.01. The small angle changes of the substrate or the subflow tube give a great influence on both the lateral and vertical growth rates. Only when the above three factors are satisfied, the subflow suppresses the thermal convection and forces the main flow to bring the reactant gas in good contact with the substrate. The surface of GaN films grown directly on Al2O3(0001) Substrate is covered with normal hexagonal-like pyramid. If one of the three factors is not satisfied, the main flow becomes a turbulent flow. (C) 2002 Elsevier Science B.V. All rights reserved.