A new plasma oxynitrocarburizing process in an air and gasoline atmosphere was developed. The influences of the atmosphere with different ratios of gasoline/air on the microstructure, surface hardness, and phase composition of 40Cr steel were characterized using optical microscope, scanning electron microscope (SEM), microhardness tester, and X-ray diffractometer (XRD), respectively. The experimental results showed that the plasma oxynitrocarburized layer was composed of bright layer and diffusion layer. It was found that the thickness, hardness, and Fe3N concentration of the plasma oxynitrocarburized layer increased with the increment of ratio of gasoline/air. (c) 2007 Elsevier B.V. All rights reserved.