In this article, we propose a new buffer structure to obtain the high quality GaN epitaxial layers grown on sapphire substrates by a separate-flow reactor of metalorganic chemical vapor deposition (MOCVD). This buffer structure consists of 200-300 Angstrom GaN nucleation layer/6 mu m GaN-bulk layer. The bulk-GaN layers have also been prepared by MOCVD. The GaN epitaxial layer grown on this buffer structure exhibits a full width at half maximum (FWHM) of double-crystal x-ray diffraction of 170 arcsec, a FWHM of 300 K photoluminescence of 56 meV, an electron mobility of 400 cm(2)/V s at 300 K and 815 cm(2)/V s at 140 K, and a concentration of 3.6 x 10(17) cm(-3) at 300 K. The GaN growth with this buffer structure has a wide growth window on the different nucleation-layer thicknesses. A good quality of GaN epitaxial layers can be obtained by using this buffer structure. (C) 1999 American Institute of Physics. [S0021-8979(99)05118-X].