9Cr18 martensite steel is commonly used as a bearing material in the aerospace industry in China. Because of its ability to treat large and irregular industrial components, plasma immersion ion implantation (PIII) is a good technique to enhance the wear resistance of 9Cr18 precision bearings to extend the working lifetime. We have recently conducted a systematic investigation to determine the process window of nitrogen PIII as well as to identify the enhancement mechanism. The surface properties of 9Cr18 steel after nitrogen PIII under different dosage and plasma conditions (filament hot electron discharge and radio frequency glow discharge) are evaluated by measuring the microhardness, wear property,: coefficient of friction, corrosion resistance, as well as elemental depth profiles. Our data indicate that the degree of improvement does not differ substantially under the various PIII conditions thereby suggesting a fairly large process window as long as enough nitrogen is incorporated tb form nitride phases.