GaN epilayers were grown on sapphire and were nitridated for various time lengths at different temperatures (0-60min, 750-850 degreesC) using the N-2 helicon wave plasma assisted evaporation deposition process. The relationship between the structure of the nitridated layers and the epitaxial growth of GaN was investigated performing a synchrotron X-ray analysis. It was found that when the nitridated layer had the highest quality crystallinity, the GaN overlayer grown on the nitridated layer showed greatly improved crystalline and luminescent properties. Although the amount of compressive strain in the optimally nitridated layer was about 1%, this layer showed low in-plane mosaicity and the largest domain size. However, excessive nitridation causes strain relaxation by reduction of the lateral domain size and the increase of lateral mosaicity, resulting in the degradation of GaN overlayers. Therefore, the adequately nitridated layer with compressive strain is a better provider of nuclei of subsequent nitride overlayers than the fully relaxed layer.