Nitridation mechanism of GaAs (001) surface is studied with systematically changing the nitridation temperature. Consequently, N1s spectrum is found useful to determine the degree of the nitridation. N1s spectrum consists of N‐Ga component and other component (denoted by component A). The N‐Ga component becomes a major component in the samples nitridated at high temperature especially without As supply. The thickness of nitridated layers was also estimated by the concentration of N atoms accumulated at the interface of GaAs/GaN/GaAs multilayers. It was changed from 1.3 to 6.6 ML with changing nitridation temperature from 346 to 571°C. The temperature dependence of the N‐Ga component coincides with the thickness of nitridated layers. This indicates that N atoms bonded with the Ga atoms during the nitridation stayed at the interface and formed a GaN thin layer by growing a GaAs capping layer on it. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)