In-situ spectroscopic ellipsometry (SE) was applied to metal-organic vapor phase epitaxial (MOVPE) growth of GaN. The effects of MOVPE reactor pressures on the growth modes of GaN were investigated by SE with the assistance of ex-situ scanning electron microscopy (SEM). We further demonstrated the on-line control of the GaN growth mode in MOVPE, which established a feasibility to grow high-quality GaN layers on sapphire substrate at the reactor pressure as low as 20 Torr. (C) 2004 Elsevier B.V. All rights reserved.