GaN epitaxial layers were grown on sapphire substrates in a separate-flow reactor by metalorganic chemical vapor deposition. The flow-rate ratio of H-2 on the upper stream to NH3 on the bottom stream is varied from 0.5 to 2. The growth condition and characterization of the GaN epitaxial layers are investigated in detail. The H-2 flow rate of the upper stream strongly affects the reactant gas flow pattern near the substrate surface and thus influences the quality of epitaxial layers. At the optimum H-2/NH3 flow ratio of 1.0, we can obtain a good quality of GaN epitaxial layers which exhibit a strong near band-edge emis-sion in the 20 K photoluminescence (PL), a full width at half maximum of 66 meV for the 300 K PL, an electron mobility of 266 cm(2)/V-s and concentration of 1 x 10(18) cm(-3) at 300 K.