Abstract: The influence of oxygen, temperature, and Bi on the morphology of MnS inclusions in free-cutting steel was studied using thermal experiments, chemical analysis, scanning electron microscopy (SEM) research, and thermodynamic calculations.The results show that high oxygen content, high temperature heating, and Bi can effectively reduce the aspect ratio of inclusions in steel and improve the proportion of large particle size inclusion.Statistical analysis reveals that the aspect ratio of MnS inclusions reduce when the total oxygen content becomes higher.The aspect ratio of inclusions increases as the area of a single inclusion in as-cast steel or rolled wire increases.With the increase of the heat treatment temperature, large particle inclusions were found in samples, and the contour of inclusions became clearer.The aspect ratio of inclusions with an area of 0-5μm²became less than 2.0 after 900℃and1200℃heating, which was lower than the aspect ratio before heat treatment by 43%.Thermal experiments reveal that in S-Bi freecutting steel with total oxygen (TO) content of 2.2×10^-5and Bi content of 0.0010%, the aspect ratio of inclusions with area of 0-5μm²is less than 1.6, which is 20%lower than that after 900℃and 1200℃heat treatment.