AISI 304 austenitic stainless steel was carburized at low temperature by means of the active screen plasma carburizing (ASPC). A layer of carbon supersaturated solid solution (i.e. the Sc phase) can be formed on the surface without precipitation of chromium carbide. The hardness of the carburized steel is greatly improved without degradation of its corrosion resistance. ASPC also solves some problems associated with the conventional DC plasma carburizing (DCPC), such as temperature uniformity, edge effect and so on. Particles sputtered from the active screen were collected and analyzed. XRD and SEM analysis indicated that the sputtered particles in sub-micron scale were neutral Fe3C and Fe2C5. The particles play the role of the carbon carrier in ASPC. Therefore, ASPC is also a multi-stage process, involving sputtering, physical adsorption, desorption, diffusion and deposition, the same as active screen plasma nitriding.