Abstract: The effect of oxygen content on the morphology of sulfides was investigated by experiment. Experimental results show that the morphology of MnS inclusions changes from Type 11 to Type I with increasing oxygen content, the diameter and area fraction of MnS inclusions increase, but the number and mean aspect ratio of MnS inclusions decrease. The influence mechanism of oxygen content on the morphology of sulfides was studied on the basis of the mathematic model, the Fe-Mn-S ternary system diagram and the Fe-Mn-S-O quaternary system diagram. In the case of the oxygen mass fraction up to 0.022%, the liquid low-melting oxides rich with MnO were found at the early stage of solidification, these oxides made the formation behavior of MnS from eutectic reaction to monoteetic reaction, and Type I MnS formed. In the case of the low oxygen content less than about 0.01%, little liquid low-melting oxides rich with MnO were found at the early stage of solidification, most of MnS precipitated in an euteetie mode at final solidification, and Type Ⅱ MnS formed.