The GaN formation on GaAs(100) using NH3 has been investigated using synchrotron radiation photoemission spectroscopy and an atomic force microscope (AFM). This study showed the effect of nitridation temperature on the compositional change, the chemical states, and the surface morphological changes in the nitridated surface layer. It was observed that ammonia is decomposed to an activated nitrogen atom above 700°C forming GaN on the surface. A thermally nitridated layer was composed of metallic Ga and GaN islands elongated along the  direction to relax the tensile strain in the  direction. As the nitridation temperature increased, the composition of GaN increased in the nitridated layer due to the efficient thermal decomposition of NH3 and the subsequent incorporation of the N atom to the metallic Ga. The surface morphology of the nitridated layer was also sensitive to the nitridation temperature and a smooth surface morphology could be obtained on the surface nitridated at 700°C.