Plasma source ion nitriding is a new approach for surface modification of steel with certain advantages over conventional plasma nitriding and plasma-based nitrogen ion implantation. A nitriding apparatus based on an electron cyclotron resonance (ECR) microwave plasma source has been developed. Nitrogen ions are accelerated from the ECR microwave plasma by a low-pulsed negative bias (typically -2 kV) which is applied directly to the workpiece, implanted, and finally diffused into the steel at elevated temperatures that are regulated up to 550 degrees C by an auxiliary heater. An application of plasma source ion nitriding into 1Cr(18)Ni9Ti austenitic stainless steel is described. The nitriding layer thicknesses are varied continuously from 0.8 to 10 mu m at various process temperatures from 230 to 480 degrees C for a nitriding time of 4 h. A dramatic increase in microhardness has been observed. Glancing angle x-ray diffraction has been used to determine the structural changes that occur in the nitrided surface layer.