Cross-sectional transmission electron microscopy (XTEM) and nano-beam electron diffraction (NBD) were used to investigate the surface microstructure of 316 stainless steel, implanted with high doses of nitrogen ions at 350°C, and 450°C using plasma immersion ion implantation (PI3). An amorphous layer up to 3 μm thick was formed on the specimen suface. With proper control of nitrogen concentration during implantation, this thick amorphous layer remains stable up to 450°C. The formation of this layer is contrary to the prevailing concept that the amorphisation of ion implanted steel results primarily from the damage caused by ion bombardment. It is proposed that solid state chemical reaction and the poor mobility of the reactant atoms are the key factors for solid state amorphisation by ion implantation.