The microscopical evolution of GaN grown by metalorganic vapor-phase epitaxy is investigated at all its stages, as nucleation layer growth, recrystallization. epitaxial overgrowth and coalescence, with the help of in situ normal incidence reflectance measurements. Sample morphology was ex situ characterized at each stage by atomic force microscopy. These investigations revealed that the nucleation layer structure, determined by the V/III precursor ratio and the reactor pressure, strongly influences the coalescence of the subsequent grown film. Particularly, in low-pressure growth additional control of the coalescence process can be gained by adjusting the initial V/III ratio of the high-temperature epilayer growth,