Abstract: The reasons for lamination defect which appeared at different positions of the pierced P91 steel billet were analyzed.The results show that the lamination defect was caused by the foreign composite inclusions containing elements such as Al,Si,Ca,O,which distributed at different positions of the steel billet continuously.During the following heat treatment,a large deformation could be caused by the external stress,and then the inclusions would have a circumferential,tangential and radial extending and become a large area,which would lead to the lamination defect.