Nitridation of sapphire (0001) substrate by plasma-excited N-2 for organometallic vapor-phase epitaxy (OMVPE) of GaN is investigated. The surface structure of nitrided layer and the relationship between the nitridation conditions and the crystalline quality of GaN epitaxial lag er are investigated The substrate surface becomes amorphous by the nitridation. An optimized nitridation improves the crystalline quality of the GaN epitaxial layer and is effective to decrease the growth temperature. A clear and transparent GaN epitaxial layer with a mirror-like surface can be grown at temperature as low as 550 degrees C. Room temperature photoluminescence indicates no deep level emission but only a strong band-edge emission from the GaN layers grown at 680 degrees C.