The isothermal decomposition characteristic of the homogeneous high nitrogen austenitic samples prepared by a new multi-stage nitriding process was investigated by SEM and TEM in this paper. Lamellar-structure precipitations arranged on the decomposed austenite grain boundaries (GBs) and the flaky gamma' particles and network-structure precipitations appeared inside of the gamma matrix. The extra high Vickers hardness more than 800 HV was found in the 5-h aged samples, which was different from those of the bainitic/martensitic structures in Fe-C alloys. The SAED analysis indicates the gamma' has the coherent relation with the parent gamma-Fe[N] phase and the interstitial nitrogen atoms are inclined to aggregate on {110}gamma'/gamma planes, which also contributes to the hardness of the matrix. (C) 2008 Elsevier B.V. All rights reserved.