The GaN epilayers are grown on (0 0 1)GaP substrates by low-pressure metalorganic vapor-phase epitaxy using the three-step growth method, which includes the growth of the GaN buffer, interlayer and high-temperature epilayer. From atomic force microscopy examinations, it is indicated that the surface roughness of the GaN buffer layer grown at 515 °C increases drastically with increasing the annealing temperature from 700 °C to 850 °C. The thickness and growth temperature of the GaN interlayer were optimized based on the X-ray and morphology measurements. It was found that a 0.2-μm-thick GaN interlayer grown at 750 °C can efficiently restrict the desorption of P atoms from the GaP surface, which is an essential step for the subsequent growth at high temperature (900 °C). Furthermore, as the epilayer thickness increased, the GaN surface became rougher due to the increase in the composition of the hexagonal component. The 77 K photoluminescence spectrum of the mirror GaN epilayer (0.6 μm in thickness) exhibits a near-band-edge emission peak at 3.36 eV as well as a yellow emission at 2.29 eV. The corresponding electron mobility and carrier concentration at 300 K were 15 cm2/Vs and 6.7×1018 cm-3, respectively.