The thermal nitridation of GaAs(100) with ammonia was investigated by synchrotron radiation photoemission spectroscopy and atomic force microscope. This study revealed the chemical composition, chemical states, and morphological changes in the nitridated surface layer. We observed that ammonia can be decomposed to an activated nitrogen atom above 700 degrees C forming GaN an the surface. Thermally nitridated layers were composed of metallic Ga and GaN islands that are elongated along the [011] direction to relax the tensile strain in the [011] 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 into the metallic Ga. The surface morphology of the nitridated layer, on the other hand, became substantially roughened at higher temperatures.