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 degreesC increases drastically with increasing the annealing temperature from 700 degreesC to 850 degreesC. 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-mum-thick GaN interlayer grown at 750 degreesC 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 degreesC). 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 mum 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 cm(2)/Vs and 6.7 x 10(18) cm (-3), respectively.