Abstract: The micro-structure of the treated layer of 17-4PH stainless steel（SS） by the QPQ（Quench-Polish-Quench） complex salt bath heat-treatment was investigated,using X-ray diffractometer（XRD） for microstructure analysis and the scanning electronic microscopy（SEM） with EDS（energy dispersive spectrum） for the profile of carbon,nitrogen and oxygen of the treated layer.The results show that the QPQ heat-treated layer is approximately 60μm,of which the surface zone was the oxide Fe₃O₄;the subsurface zone is the compound ξ-Fe₂（N,C） and the inner zone is the mixture of CrN and αN.The compound nitride γ′-Fe₄N exists value at subsurface.After the oxidation layer was removed,the micro-hardness of the treated layer is ob-served to be first increasing and then declining with the depth increasing.between 15μm and 25μm.The interface between the oxide（magnetite） and the compound γ′-Fe₄N was sup-posed to be the place of the ξ-Fe₂N（N,C）_1-x nuclei.The oxygen concentration reveals the highest value at approximately 3μm from the surface,while the nitrogen and carbon concentration orderly reaches the high peak