Cross-sectional transmission electron microscopy (XTEM), selected area diffraction (SAD) and nano-beam diffraction (NBD) techniques were used to investigate the surface microstructure of 316 stainless steel, implanted with high doses of nitrogen ions at 150, 250, 350, 450 and 520°C using plasma immersion ion implantation, It has been found that the treatment temperature has a strong influence on the evolution of the microstructure. An amorphous layer of about 1 μm thick with a heavily stressed substrate underneath was observed on the 150°C implanted sample. Both the 250 and 350°C implanted samples showed a thin nanocrystalline sublayer at the outermost surface and an amorphous sublayer between the nanocrystalline sublayer and the substrate. A thick amorphous layer up to 3 μm thick was formed on the 450°C implanted sample whereas at 520°C, cellular precipitation of CrN and α-ferrite dominated the system. It is suggested that a solid state chemical reaction and the poor mobility of the reactant atoms are the key factors for the solid state amorphisation by nitrogen ion implantation into austenite.