Abstract: The 1kg crucible experiment was carried out in laboratory. The shape, size and composition of inclusions in medium-carbon high-sulphur structural steel were investigated. The results show that fusiform inclusions, which can improve the cutability of the steel, account for 68.11% in total inclusions, and the ratio of fusiform inclusions increases with increasingCa/S. After calcium treatment the inclusions is very tiny, inclusions of less than 2.5 μm in size account for 76.05% and scattered in the matrix of the steel. The inclusions are mainly composed of (Mn, Ca)S sulfide and complex inclusions in which the core is calcium aluminate and the shell is sulfide. There are small amount of complex inclusions of calcium aluminate and calcium sulfide in the inclusions, too. The cutability of the ingot steel containing sulphur and calcium is better than that of middle carbon steel.