Abstract: The morphology,size,and composition of inclusions were studied by scanning electron microscopy(SEM) in pipeline steel during refining process. The Ca-O equilibrium curve,Ca-S equilibrium curve,and the stability diagrams of inclusions(CaO and CaS) were discussed to explain the formation mechanism of CaO and CaS complex inclusions. The results show that the excessive addition of Ca-Si wireis the reason causing the formation of CaO and CaS complex inclusions.