Abstract: Medium carbon non-quenched and tempered steels with different S contents were smelted with a vacuum induction furnace. Several experiments were conducted to study the effect of S content on the morphology of sulfides and microstructure in the steels. The results show that with the increase of S content from 0.025 wt% to 0.065 wt%, the number of sulfide inclusions increases, and the aggregation of sulfide inclusions is promoted, while the microstructure becomes finer due to the increasing area ratio of intragranular ferrite. Thermodynamic calculations and phase diagram information were employed to explain the transition of MnS from type Ⅱ to type m. The angular type Ⅲ MnS is resulted from divorced eutectic reaction before the solidification completes, and the rod-like type Ⅱ MnS is derived from normal eutectic reaction at the end of the solidification process. Both MnS and MnS-V (C, N) complex inclusions are found to act as effective nucleation sites to improve the formation of intragranular idiomorphic ferrite.