The lattice constants of GaN layers along the a-axis were estimated using reflection high-energy electron diffraction (RHEED) patterns during metal-organic molecular beam epitaxy (MO-MBE) growth. The lattice relaxation of GaN layers was observed during the initial growth. The crystal structure changed from the mixed (cubic and hexagonal) to the hexagonal phase at the high growth temperature of 814 degrees C. On the other hand, the crystal structure changed from the hexagonal to the mixed phase at the low growth temperature of 772 degrees C.