Abstract: The formation behavior of sulfides in free-cutting steel was investigated under unidirectional solidification.The relations of the dendrite arm spacing and sulfides' average diameter to solidification conditions were established.Experimental results show that the average diameter of MnS inclusions decreases with increasing temperature gradient around the solid/liquid zone and casting speed,but the number of MnS inclusions increases.The complete precipitation temperature of MnS is about 100℃ lower than the solidus temperature.The morphological changes of MnS are mainly effected by the activities of constituent elements,in other words,they are mainly influenced by interfacial energy.The relations of the dendrite arm spacing and sulfides' average diameter to solidification conditions were regressed,and the regression results are basically in agreement with the ones of previous researchers.