Abstract: To research the major type of inclusions in stainless steel slabs and find out their primary origin, the composition of the non-metallic inclusions was analyzed by SEM, and three tracer experiments were performed by adding a tracer into AOD slag, ladle slag and tundish slag, respectively. The results show that the primary non-metallic inclusions in 0Cr18Ni9 stainless steel slabs is CaO-SiO₂-Al₂O₃-MgO system inclusions, the second is MgO-Al₂O₃ spinel and sulphide; the primary origin of the non-metallic inclusions is the production of deoxidation and desulfuration during AOD reducing and the AOD slag suspension in the molten steel during tapping; after tapping, the molten steel is not obviously polluted by ladle slag, tundish covering flux and mold flux.