Plasma immersion ion implantation is a nonline-of-sight method for fabricating amorphous carbon or diamond-like-carbon coatings on steels to improve the surface properties. In this work, carbon thin films are synthesized on 9Cr18 (AISI440) stainless bearing steel by acetylene (C2H2) plasma immersion ion implantation (Pm). The effects of the processing parameters, including rf power, sample voltage pulse duty cycle, and target bias, on the structure and surface properties of the carbon thin films is systematically investigated employing Raman spectroscopy, Auger electron spectroscopy, friction coefficient measurement, and wear test. The results reveal that carbon films several hundred nanometers thick with a well-mixed interface are formed on the 9Cr18 steel after C2H2PIII but the structure and properties of the carbon films vary greatly under different PIII conditions. There is an optimal process window within which the synthesized films have superior properties, and current densities that are too high do not yield films with the desired performance.