The effect of the growth rate of the nucleation layer on the growth of a high temperature GaN layer has been studied by observing the in situ normal incidence reflectance during the growth of GaN by metalorganic chemical vapor deposition. This study revealed that the lateral growth and coalescence of a high temperature GaN layer was enhanced as the growth rate of the nucleation layer was increased. However, the measurement of (102) hk-circle scan using x-ray diffraction showed that the: in-plane structural qualities of GaN were found to be deteriorated by increasing the growth rate: of the nucleation layer. The electron mobility was also decreased by increasing the growth rate of the nucleation layer. These results suggest that the nucleation sites are well oriented on the sapphire substrate at a low growth rate of the nucleation layer but that the lateral growth and coalescence of the GaN layer is hindered due to the limited surface diffusion of adatoms at a fast growth rate df the nucleation layer.