The role of temperature ramping rate during the two-step growth of GaN-on-sapphire by metalorganic chemical vapor deposition is explored. The surface morphology and crystalline properties of the low-temperature-deposited GaN buffer layer annealed under various ramping rates (20-60 degreesC/min) to 1000 degreesC were investigated by atomic force microscopy and X-ray measurements. For lower ramping rates employed, a dramatic re-evaporation of the GaN buffer layer might occur. This makes the buffer layer thinner, yielding a GaN epilayer of hexagonal morphology. However, as the higher ramping rates are applied, the surface becomes rougher and exhibits hexagonal three-dimensional islands. It could be due to the fact that the grains of the GaN buffer layer do not have enough time to coarsen. Under a temperature ramping rate of 40 degreesC/min, a smooth buffer-layer surface can be maintained and results in a subsequent high-quality overlayer deposition. The mirror GaN epilayer shows a near-band-edge peak (25 K) centered at 3.477eV with a full-width at half-maximum as narrow as 13.1meV. The observed temperature-ramping-rate effects can be interpreted by the coalescence mechanism of the GaN buffer layer.