This article reports the morphology, size, and distribution evolution of Mn S inclusions in non-quenched and tempered steel during heat treatment. The variation of single large-sized Mn S inclusions at high temperature was observed in situ using a confocal scanning laser microscope(CSLM). The slender Mn S inclusions first changed to pearl-like strings. These small-sized pearls subsequently coalesced and became closer together as the temperature increased. Large-sized Mn S inclusions in non-quenched and tempered steel samples with different thermal histories were investigated with respect to the evolution of their morphology, size, and distribution. After 30 min of ovulation at 1573 K, the percentage of Mn S inclusions larger than 3 μm decreased from 50.5% to 3.0%. After a 3 h soaking period, Ostwald ripening occurred. Most Mn S inclusions moved from the grain boundaries to the interior. The present study demonstrates that heat treatment is an effective method of changing the morphology, size, and distribution of Mn S inclusions, especially large-sized ones. This article reports the morphology, size, and distribution evolution of Mn S inclusions in non-quenched and tempered steel during heat treatment. The variation of single large-sized Mn S inclusions at high temperature was observed in situ using a confocal scanning laser microscope ( These small-sized Mn s inclusions in non-quenched and tempered steel samples with different thermal histories were investigated with respect to the evolution of their morphology, size, and distribution. After 30 min of ovulation at 1573 K, the percentage of Mn S inclusions was more than 3 μm decreased from 50.5% to 3.0%. After a 3 h soaking period, Most Mn S inclusions moved from the grain boundaries to the interior. The present study demonstrates that heat treatment is an effective method of changing the morpholog. y, size, and distribution of Mn S inclusions, especially large-sized ones.