Abstract: The change process of MnS inclusions in YF45MnV steel was in-situ observed by confocal laser scanning microscope. The result shows that during the continuous heating process, slender MnS inclusions firstly change into string-like inclusions composed of subsphaeroidal sulfides, then these subsphaeroidal sulfides diffuse and grow up with the number decreasing. When the temperature increases to some extent, MnS inclusions dissolve to the steel and the areas of corresponding sulfides decrease.