GaInN/GaN layers with low-temperature (LT) AlN buffer layers were grown on sapphire substrates by OMVPE. Two types of sample series were investigated by X-ray CTR and X-ray reflectivity measurements. One of them was grown on a sapphire substrate with nitridation process before LT-AlN buffer layer growth. The other was grown without the nitridation process. The X-ray CTR and X-ray reflectivity measurements show that 1) an amorphous layer was formed on the sapphire substrate by annealing at 1150^oC in H₂, 2) when sapphire was exposed to NH₃ at temperatures lower than 800^oC, formation of the amorphous and/or crystalline AlN layer did not occur, 3) nitridation process caused a crystalline AlN layer growth on the sapphire substrate and an amorphous AlN layer of 100∼50Å on it, 4) the crystalline AlN layer formed by the nitridation process worked as a seed crystal to make the LT-AlN layer high crystalline, 5) since the high crystalline AlN layer formed by the nitridation process did not work well as buffer layer, the crystalline quality of GaInN and GaN layers was poor, 6) since the poor crystalline LT-AlN layer obtained without the nitridation process worked well as a buffer layer, the crystalline quality of GaInN and GaN layers was good.