High-quality GaN epitaxial layers with a multiple-pair buffer layer have been grown on sapphire substrates in a separate-flow reactor by metalorganic chemical vapor deposition. Each pair of buffer layer consists of a 300 Angstrom thick GaN nucleation layer grown at a low temperature of 525 degrees C and a 1-4 mm thick GaN epitaxial layer grown at a high temperature of 1000 degrees C. The GaN samples with a multiple-pair buffer layer are characterized by double-crystal x-ray diffraction (DC-XRD), Hall method and photoluminescence (PL) at 300 K, and etch-pit density measurements. The optimized condition to obtain the best quality of GaN epitaxial layers is to grow the four-pair buffer layer with a pair thickness of 4 mm. The GaN samples with the optimized buffer layer exhibit a narrow full width at half maximum (FWHM) of 150 arcsec and a strong intensity in DC-XRD, a high electron mobility of 450 cm(2)/V s, a low background concentration of 3X10(17) cm(-3), a low etch-pit density of mid-10(5) cm(-2), and a narrow FWHM of 56 meV in PL spectrum. (C) 1999 American Institute of Physics. [S0021-8979(99)02712-7].