Abstract In present work, a perfect nitrogen-expanded austenite free from the deformation defect is prepared by the plasma-based low-energy nitrogen ion implanted (also named plasma source ion nitriding) TEM foil of Fe-Cr-Ni austenitic alloys under an unconstrained condition. The effective area in nano-scale, which is constructed by the perfect nitrogen-expanded austenite without stress and plastic strain, is obtained for TEM observation. The electron diffraction directly proves that the Fe4N-like long-rang ordered transformation is the nature of nitrogen-expanded austenite only dependent on nitrogen concentration, and independent on stress or defect. Moreover, the intrinsic microstructure of the nitrogen-expanded austenite is confirmed without twins and e-martensite, which were formed from the constrained deformation of the nitrided layer depending on the stacking fault energy.